Australia–Japan Research Project at the Australian War Memorial

Army operations in the South Pacific area: Papua campaigns, 1942–1943
Chapter 4: Commencement of the overland-route offensive on Port Moresby

(A translation of Bôeichô Bôei Kenshûjo Senshishitsu (ed), Senshi sôsho: Minami Taiheiyô Rikugun sakusen <1> Pôto Moresubi–Gashima shoko sakusen (War history series: South Pacific area army operations (1), Port Moresby–Guadalcanal first campaigns) (Tokyo: Asagumo Shinbunsha, 1968): 167–230.)
Translated by Dr Steven Bullard


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Chapter contents

Chapter 4: Commencement of the overland-route offensive on Port Moresby

Ri Operation Study


Transition from an advance by sea to the advance by land
Formation of the 8th Fleet and disposition of the units
Operations of the 25th Air Flotilla
Plans of Imperial Headquarters in early August
Notes



Ri Operation Study

Commencement of research for the Ri Operation

The headquarters of the 17th Army flew out of Tokyo on 7 June 1942 and lodged at Fukuoka with the intention of flying on to Manila the following day via Taipei. That evening, however, a telegram was received from Imperial Headquarters that stated: "Delay your departure and wait to be contacted by staff officer Imoto."

At around 9 pm the following night at the Matsushima Ryokan, staff officer Imoto from Imperial Headquarters secretly informed the 17th Army staff officers of the setback in the Midway operation and the postponement of the FS Operation. Though he did not mention the real truth of the defeat, the news was a great shock to the general staff. There were none among them who had considered that the operation would be cancelled.[1]

Staff officer Imoto presented the "Outline of implementing the F Operation according to the prevailing conditions", which had been adopted by the Army Department of Imperial Headquarters. After that, he relayed the intentions of Imperial Headquarters, as follows:

According to the latest intelligence from the navy, it is felt that the strategic course of an overland attack of Port Moresby is possible. The 17th Army should use the time while the [FS] Operation is postponed to gather intelligence concerning the feasibility of this strategy.[2] The 17th Army command party left Fukuoka on 9 June and arrived at Davao on 15 June by way of Taipei and Manila. The following instruction from the chief of staff at Imperial Headquarters was received from Prince Takeda, an army staff officer, at Manila the previous day:

Great Army Instruction No. 1,179[3] At this time, Prince Takeda showed the staff of the 17th Army the account of an English explorer that indicated a road to Port Moresby. Further, the Army Department of Imperial Headquarters gained the cooperation of the navy and explained the "Outline of the Ri Operation Study" on 13 June, as follows:[4]
Units under the command of the 17th Army

The strength of the 17th Army at the time of the arrival of its headquarters in Davao on 15 June was as follows:

The South Seas Force was stationed in Rabaul after the difficulties encountered in the sea-route attack of Port Moresby. It was incorporated into the 17th Army command structure on 20 May.

The 35th Infantry Brigade (brigade headquarters and 124th Infantry Regiment, less the 114th Infantry Regiment – after that known as the Kawaguchi Detachment) participated in the invasion of British Borneo. After completion of those operations, it was transferred to the Philippines and took up subjugation duties from the beginning of April. The brigade was transferred from Cagayan (200 kilometres north of Davao) to Davao after receiving the great order of 18 May. After arriving on 6 June, the brigade was incorporated into the 17th Army command structure. The commander of the brigade was Major General Kawaguchi Kiyotake, and the chief of staff was Major Ôsone Yoshihiko.

After participating in the invasion of Java, the various units of the Aoba Detachment were deployed in subjugation duties in the western area of Java. The detachment was mobilised according to the great order of 18 May and departed from Batavia on 26 May. It landed in the Davao area (Tebunko and Lasang) on 5 June and was incorporated into the command of the 17th Army.

Operational preparation orders issued previously on 28 May required the detachment to assemble at Parao in readiness for the offensive against Port Moresby. The delay in the offensive resulted in the detachment remaining at Davao. The commander was Major General Nasu Yumio, and the chief of staff was Captain Taguchi Kazuo.

The 41st Infantry Regiment of the 5th Division participated in the Malaya campaign. It was subsequently attached to the 9th Infantry Brigade and redeployed to the Philippines, where it was assigned to subjugation duties in the southern Philippines from April. The operation was postponed while the unit was preparing to advance to Davao from Cagayan according to the great order of 15 May. Consequently, the 41st Infantry Regiment was incorporated into the 17th Army command structure on 15 June.

Imperial Headquarters had determined that the South Seas Force and the main force of the 17th Army (less the advance communications units in Rabaul) would be incorporated into the 17th Army command when they arrived in Davao. However, units that had not arrived in Davao by 12 June were also incorporated from 15 June at their present locations. The commander of the 41st Infantry Regiment was Colonel Yazawa Kiyomi. The two battalions under his command were called the Higashi Detachment according to the operational preparation orders issued on 28 May. The other two battalions were attached to the command of the 35th Infantry Regiment.

The 15th Independent Engineer Regiment had arrived previously in Davao and was incorporated into the 17th Army command.

Command of the 17th Army had units stationed on Mindanao, with some at Davao and Rabaul. On 17 June, 17th Army headquarters ordered units to deploy to the following locations to prepare for operations.

17th Army headquarters was to be located at Davao. It arrived by the high-speed Ayatosan Maru on 22 June.
UnitLocation
35th Infantry Brigade (less two battalions seconded)Parao
South Seas ForceRabaul
Aoba DetachmentDavao
Higashi Detachment (two battalions of 41st Infantry Regiment returned)Davao
Army unitsDavao

These orders resulted in the redeployment of the 35th Infantry Brigade and the Aoba Detachment. The 35th Brigade was intended for the Fiji offensive operations. Parao had numerous coral shelves that enabled the brigade to practice landing operations.[5] In accord with these orders, the brigade left Davao on 2 July and arrived in Parao on 4 July. The Higashi Detachment arrived in the Davao area (Dariaon) from Cagayan on 28 June.

The main basis for the activities of the 17th Army lay in the Ri Operation Study issued by the chief of staff. At this time, Imperial Headquarters was leaning heavily towards an overland offensive against Port Moresby owing to the expected results of the Ri Operation Study.

Conception of the advance by land

It was natural that the reduction of air power and strategic limitations based on the ability to directly apply military power on the battlefield were also influential on the success or failure of any overland offensive. At that time, however, the dominant concerns were topographical: could a land army advance overland from eastern New Guinea and approach Port Moresby from the rear?

The highest peaks of the Owen Stanley Range that run along the spine of eastern New Guinea are Mt Albert Edward (75 kilometres south-west of the mouth of the Mambare River) and Mt Victoria (50 kilometres further south of Mt Albert Edward). Both peaks are over 4,000 metres above sea level and are covered with snow year-round. In the upper reaches of the Mambare River valley to the south-east of the foothills of Mt Victoria lies the plateau called Kokoda. The Owen Stanley Range is narrowest to the south-west of Kokoda. If the range could be traversed at this point then the prize of Port Moresby would be ready for the taking.

Immediately after the battle of the Coral Sea, the navy’s 8th Base Force and the South Seas Force at Rabaul continued investigations from documents and local testimony into the existence of roads across to Port Moresby. Initially, overland supply routes were sought after the revival of the sea-route invasion of Port Moresby.[6] As a result of this, an early report was submitted to Imperial Headquarters that there seemed to be a road to Port Moresby.

Investigations by Imperial Headquarters and local units up to the end of June was not conclusive concerning the track to the west of Kokoda, but showed that it divided en route into two pack-horse trails and two other narrow trails that led to Port Moresby. The existence of the following from the coast to coast to Kokoda was determined: a narrow trail along Mambare River valley up to Kokoda (native trail); a narrow trail along Kumusi River valley up to Kokoda (native trail); a pack-horse trail from Buna to Kokoda. Meanwhile, two reconnaissance aircraft from the 25th Air Flotilla had discovered a road between Buna and Kokoda on 27 June that could accommodate motor vehicles. Another reconnaissance plane was despatched on 30 June escorted by four Zero fighters. As a consequence of these investigations, the 25th Air Flotilla telegraphed the following fighter report (no. 82) to related units:
Reconnaissance on 3 July added to this intelligence the existence of a simple wooden bridge to the south-west of Kokoda. It was barely 1.5 metres wide and was deemed unsuitable for trucks. (The height of reconnaissance was 500 metres.) While sufficient investigation of these roads was considered essential, an overland offensive was now considered possible. Further, the central authorities, including the emperor, had taken a keen interest in this intelligence through special orders issued on 1 July by chief of staff Sugiyama.

Imperial Headquarters requested a force to occupy the banks of the Mambare River according to the results of the Ri Operation Study. This was a decision to advance a so-called "bridgehead reconnaissance force". With information on the existence of the previously mentioned roads, 17th Army headquarters despatched this reconnaissance bridgehead not to the banks of the Mambare River, but to Buna.

Issues related to the selection of units

The next problem was the selection of units to undertake the study. The headquarters of the 17th Army considered assigning the main body of the 15th Independent Engineer Regiment, which had performed with distinction in the Malaya campaign, together with elements of an infantry unit. Whether this infantry unit should be the South Seas Force or the Aoba Detachment was to be determined by future deployment concerns.

As previously described, the South Seas Force was initially intended for the sea-route offensive against Port Moresby, but was transferred to the capture of New Caledonia under the FS Operation. The Aoba Detachment was planned for use in the offensive against Port Moresby. Both detachments had been informed of these arrangements and had begun appropriate preparations.

After the difficulties they encountered in the sea-route offensive against Port Moresby, the commander and staff of the South Seas Force were enthusiastic about the prospects for the New Caledonia operations, and had lost interest in the Port Moresby offensive.[7] It was natural, therefore, for the 17th Army headquarters to consider assigning the Aoba Detachment as infantry support to the 15th Independent Engineer Regiment on the Port Moresby operation as planned.

However, there were considerations over and above the condition of the despatched units. There was advantage in using the South Seas Force stationed in Rabaul over the Aoba Detachment in Davao if rescue was considered necessary, or if an overland offensive against Port Moresby was considered urgent. For the Aoba Detachment to be deployed, the main strength of the unit would need to be transferred to Rabaul, where accommodation was limited and losses from enemy air attacks would need to be considered. This was not considered the best course of action.[8]

The 17th Army command risked the possibility of failure of the overland offensive against Port Moresby if it assigned the responsibility for the Ri Operation Study to the South Seas Force, whose command had not shown enthusiasm for the task. Further, the psychological impact of suddenly changing arrangements and month-long preparations were given great consideration, so the plan to assign the Ri Operation Study to the Aoba Detachment was kept in reserve.

At this time it had become clear that Imperial Headquarters could barely supply one vessel for use in the Ri Operation Study. The availability of suitable shipping had not been an issue in the abovementioned consideration concerning the deployment of the Aoba Detachment. Without delay, the 17th Army command once again assigned the Port Moresby offensive to the South Seas Force, and consequently entrusted the unit with undertaking the Ri Operation Study. The commander of the 17th Army summoned the commander of the South Seas Force to Davao in order to communicate his responsibilities directly and to urge him to undertake his duties with resolve.[9]

Commander Horii and staff officer Tanaka of the South Seas Force arrived in Davao on 30 June. Commander Horii’s opinion was sought on the feasibility of the overland offensive based on the research done to that time. As expected, he was unenthusiastic about the overland offensive, stating that it would be extremely difficult with a high risk of failure. His reasons were as follows:[10]
However, as this conclusion was based on the calculations of the South Seas Force headquarters, even commander Horii did not strongly press his objections at the time.

Orders from the 17th Army

17th Army headquarters issued the following orders on 1 July:
Instructions from the chief of staff concerning these orders were as follows:
Army and navy local agreement

Agreement was reached at Truk on 4 July 1942 between the 17th Army and the 4th Fleet concerning the strategy of the Ri Operation Study. Prior to this, the navy had determined that responsibility for the execution of the Ri Operation Study would lie with the 4th Fleet. This was an express command issued by the commander of the Combined Fleet, as contained in the following instruction on 11 June concerning the study: "Primary responsibility for the Ri Operation Study lies with the commander of the 4th Fleet." Consequently, because the study was solely the responsibility of the 17th Army and the 4th Fleet, it was no more than simply a general cooperative relationship with the 11th Air Fleet (the air unit in the area).

With the Buna area only 20 minutes flying time from the airbase at Port Moresby, there were still concerns for the 17th Army units advancing by sea. The 25th Air Flotilla of the navy, however, made provision for staff officer Miyazaki Atsushi to be despatched as an observer.

Further, the 17th Army considered that there should be continuity between operations to undertake research for the Ri Operation, and any overland offensive strategy which developed from the research itself. Consequently, there was a desire to make reference to the Port Moresby offensive even with the cooperative agreements with the 4th Fleet. At the same time, the navy was reorganising its order of battle for units in the South-East Area of operations. After mid-July, the 8th Fleet, and not the 4th Fleet, was expected to be the formation actually involved in the operation. Consequently, the 4th Fleet could not reach agreement on any offensive strategy outside of its responsibility: that is, beyond the early phase of the Port Moresby campaign. Operational agreements for the Port Moresby offensive were therefore not all-encompassing, and until the first official telegram agreements from the 8th Fleet, consisted of no more than responses between the two staffs.[11]

In accord with these agreements, the commander of the 4th Fleet despatched units for the Ri Operation Study to Rabaul. These units were commanded by Rear Admiral Matsuyama Kôji and comprised the 18th Squadron (Tenryû and Atsuta) and the 29th Destroyer Squadron (Asanagi, Yûzuki, and Uzuki). These units arrived in Rabaul on 9 July and continued preparations for the operation.

Deployment of the Yokoyama Advance Party

The commander of the South Seas Force presently returned to Rabaul and allocated the main strength of the 15th Independent Engineer Regiment, the 1st Battalion of the 144th Infantry Regiment, and the 1st Company, 1st Battalion of the 55th Mountain Artillery Regiment to the advance force. This was to be called the Yokoyama Advance Party and was commanded by Colonel Yokoyama Yosuke of the 15th Independent Engineer Regiment.

The two greatest issues facing the South Seas Force in the despatch of the advance party were supplies and anti-aircraft strategies. There was no other option than to rely on the 25th Air Flotilla for anti-aircraft protection, but preparations and training were required in the case of transports being sunk prior to disembarkation at Buna. To implement a supply policy, changes were made to organisation and equipment. The mountain artillery company had barely one mountain gun and was limited to 200 rounds carried by individual soldiers in backpacks for ease of transport.

The increase in the number of carriers was an unavoidable problem. The navy’s 8th Base Force agreed to release approximately five hundred members of the Formosan Takasago Volunteers and Korean labourers to the South Seas Force. Both groups were attached at the time to the navy.

Approximately two thousand natives were also commandeered. This action had unfavourable repercussions for occupation administration and for later attempts to indoctrinate and pacify the local populace. Operational demands, however, took precedence.[12]

Detailed agreements were made between the South Seas Force and the 18th Squadron and 25th Air Flotilla from the navy’s 8th Base Force on 13 July.[13] Although preparations were almost complete for despatch of the advance party, the main elements of the unit had not yet arrived in Rabaul.

The main strength of the 15th Independent Engineer Regiment and the 47th Field Anti-aircraft Artillery Battalion, newly formed for inclusion in the South Seas Force, left Davao on 2 July and eventually arrived in Rabaul on 14 July. The anti-aircraft capacity of the South Seas Force had increased from the original one company at the start of the war to two companies.

The commander of the South Seas Force issued orders ("Horii operations orders B, no. 85") to the Yokoyama Advance Party on 14 July. These orders made clear to the advance party that, in addition to simply investigating the advance route, it should "Carry out partial preparations with a view to the operations of the main strength of the force."[14] The movement of the advance party in compliance with these orders was made clear as follows:
Orders concerning repair to roads and the stockpiling of ordnance appeared to be preparations in anticipation of the overland offensive by the main strength of the force. Consequently, the Yokoyama Advance Party, as the name suggests, was not so much deployed to reconnoitre the possibility of an overland attack, but must be recognised as a unit deployed to prepare for an overland attack.

However, the decision to conduct an overland offensive, which was a burning issue for Imperial Headquarters at that time, was probably waiting for a judgment by the commander of the engineer regiment on the front line.

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Transition from an advance by sea to the advance by land

Commencement of the advance by land

Imperial Headquarters officially cancelled the FS Operation on 11 July. The orders for the 17th Army to change duties had not yet been discussed. These had been received by telegram by 17th Army headquarters at Davao before 14 July. However, Imperial Headquarters had withheld operational outline instructions for the new responsibilities of the 17th Army, no doubt because they were waiting for the results of the Ri Operation Study before issuing definitive orders.

On 15 July, an Imperial Headquarters army staff officer, Tsuji Masanobu, arrived at 17th Army headquarters in Davao. His despatch was a result of the decision to cancel the FS Operation on 11 July. Staff officer Tsuji and the 17th Army chief of staff, Futami, had both been attached to the Army General Staff 3rd (War Mobilisation) Section, and had been friends for some nine or ten years.

Staff officer Tsuji discovered that there were female typists within the army headquarters and recommended to chief of staff Futami that they be immediately repatriated to Japan, to which he agreed straight away.[15] This was a trivial issue, but unexpectedly for the high command of the 17th Army, Tsuji informed them that day that Imperial Headquarters had already decided to undertake the overland offensive.

Lieutenant Colonel Tsuji delivered them a great order dated 11 July, and after he explained the conditions of the navy in particular in cancelling the FS Operation, he stated the following:

In order to effectively carry out an air war of attrition against eastern New Guinea, it is imperative that Port Moresby be attacked as soon as possible. Even the emperor is particularly concerned about this issue. Therefore, without waiting for the results of the Ri Operation Study, Imperial Headquarters has ordered the 17th Army, by this great order, to attack Port Moresby. It is expected that army and navy central agreements concerning this will be telegraphed to Rabaul and Davao no later than 24 July. The Ri Operation must now be executed without the feasibility study. I would like the 17th Army to proceed with local agreements with the navy and begin the offensive against Port Moresby. I would like this achieved with all haste and with firm resolve.[16] The 17th Army was at this time in preparations to conduct the Ri Operation Study, with the Yokoyama Advance Party to land at Buna one week later.

However, a great order had already been issued. Prior to the issue of the great order, the plan had been to issue operational outlines after the results of the Ri Operation Study were known. It was not improbable that Imperial Headquarters would then not wait for the study results and decide on the overland attack option.

17th Army Headquarters confirmed the intent of Imperial Headquarters through staff officer Tsuji, and immediately decided to begin the overland offensive. Operational outlines were quickly drawn up on 15 July.[17]

However, all of this was an independent decision by Imperial Headquarters staff officer Tsuji. Imperial Headquarters were certainly leaning towards the overland offensive, but they had in fact not changed their position to wait for the results of the Ri Operation Study before making a determination. The first the 17th Army chief of staff knew of this was on 25 July when he received a telegram from Colonel Hattori, the head of the Operations Section in Imperial Headquarters, stating that he was "waiting for the results of the 17th Army study".[18]

Senior staff officer Imoto, at Imperial Headquarters at this time, had the following memory of these events after the war:
Staff officer Tsuji, in addition to the previous statement, passed on the intent of Imperial Headquarters on 15 July. The main points of this were as follows:
According to the "Operations of the 17th Army at Guadalcanal", the following exchange took place between the general staff of the 17th Army.[19]
Previously, on 18 May, when the battle order for the 17th Army was issued, the shipping transport headquarters, led by the transport communications commander (Imperial Headquarters Army Department, head of the 3rd Department), was placed under the command of the 17th Army headquarters for the execution of the FS Operation. The shipping transport units were as follows. The independent engineer regiment was responsible for coastal landing (unloading) using large and small barges. In effect, they were shipping engineers:
Up to that time, the shipping transport units for the South Seas Force had only consisted of one company of the 10th Independent Engineer Regiment (less one platoon), elements of the 37th Anchorage Command, and a platoon strength sea duty unit. After the cancellation of the FS Operation, the abovementioned units remained under the command of the 17th Army.

The Army Department of Imperial Headquarters had made provision for approximately 100,000 tonnes of shipping for the execution of the FS Operation. With the cancellation of the operation, 50,000 tonnes was deemed appropriate for use by the 17th Army in its campaigns, with the remainder to be used in reserve as conditions dictated.

Offensive orders from the 17th Army

17th Army headquarters in Davao issued orders on 18 July for the offensive against Port Moresby. The outline of these orders is as follows:[20]
As these orders make clear, the 17th Army had planned for one part of the 35th Infantry Brigade to land to the east of Port Moresby in concert with the overland offensive of the South Seas Force. At that time, this small force had an infantry battalion as its core. It was also considered sufficient for a force of about company strength to occupy Samarai and the Louisiade Archipelago to the east.

The mobilisation of the overland offensive was a credit to the positive and prompt leadership of staff officer Tsuji. On 19 July, staff officer Tsuji transmitted the following report to Imperial Headquarters: "The Oki Unit has been issued army orders for the execution (not research) of the Ri Operation according to the central conception of the operation." This "execution (not research)" refers to the overland offensive. Whether this was precisely understood by Imperial Headquarters is open to question.

The Army Department of Imperial Headquarters added to the order of battle of the 17th Army the 52nd and 54th Independent Supply and Transport Companies from the 15th Army, and the 1st Bridging Supply Company of the 9th Division from the 23rd Army. This was made clear in contact to Tsuji from Imperial Headquarters on 20 July. It was indicated that these transfers were "for the purpose of the Ri Operation Study". On the same day, the Army Department ordered the Aoba Detachment, one company of the 2nd Tank Regiment, and one company of the 21st Field Heavy Artillery Battalion to the order of battle of the 14th Army. This was, however, with the condition that these units were to be prepared for redeployment to other areas as the conditions dictated from the middle of September.

The 17th Army made various necessary notifications to the 4th Fleet concerning previously established arrangements. The navy considered these arrangements to be part of the Ri Operation Study, as they replied, "In considering at this time the Ri Operation Study …". The army imagined that the navy had not yet received a great order for the Port Moresby offensive.

The commander and staff of the 17th Army left Parao on 22 July and arrived in Rabaul by way of Truk. Staff officer Tsuji accompanied them.

Orders from the South Seas Force

Previously, the transport of the Yokoyama Advance Party to conduct the Ri Operation Study was divided into two echelons. The first comprised the main strength of the unit and had completed its preparations by 19 July. It had planned to depart Rabaul harbour at 8 pm the following evening. Immediately prior to the departure, the aforementioned army order for the overland offensive was received. By this, the commander of the South Seas Force determined to "Prepare for an offensive on Moresby and nearby airstrips over the Buna to Kokoda road after landing the force in the Buna area." At midday on 20 July, the Yokoyama Advance Party was ordered as follows:
According to Major Koiwai, the South Seas Force at this time included the following main personnel:

Force commander: Lieutenant General Horii Tomitarô
Staff officer: Lieutenant Colonel Tanaka Toyonari
Staff officer: Lieutenant Colonel Tomita Yoshinobu
Staff officer: Major Toyofuku Tetsuo
144th Infantry Regiment commander: Colonel Kusunose Masao
144th Infantry Regiment, 1st Battalion commander: Lieutenant Colonel Tsukamoto Hatsuo
144th Infantry Regiment, 2nd Battalion commander: Major Horie Tadashi
144th Infantry Regiment, 3rd Battalion commander: Lieutenant Colonel Kuwada Genjiirô
55th Mountain Artillery Battalion commander: Lieutenant Colonel Hozumi Shizuo

Preparedness of the navy

At that time, the 4th Fleet had planned for units under the command of the 5th Yokosuka Naval Landing Party to land in the Buna area at the same time as the landing of the Yokoyama Advance Party. This was to establish a landing outpost and construct an airbase. Even local navy units, in line with the army, considered these to be more than simply reconnaissance operations, but were measures to establish advance air and ground bases in preparation for an assault on Port Moresby. This way of thinking did not indicate a "determination for an overland route" by the army, but rather recognised the need for advance airbases in the Buna area even if the overland route was considered too difficult and the sea route was adopted after all.

The 4th Fleet, in cooperation with the 17th Army, provided transport and protection for the Yokoyama Advance Party, and mobilised the following units for the invasion in the Buna area:

FormationCommandStrengthResponsibility
Main force18th Squadron commander (Rear Admiral Matsuyama Kôji)18th Squadron
(Tenryû, Tatsuta)Maintain supply line for entire operation
1st Security DetachmentTsugaru commanderTsugaru, No. 32 Submarine ChaserEscort and protect transport convoy
2nd Security DetachmentAsanagi commanderAsanagi, Yûzuki, UzukiSea-route guidance
Sea Mobilisation UnitAir unit commanderThree observation planes from Kiyokawa MaruAnti-submarine, sea-route guidance, cooperation with landing operation
Landing UnitSasebo Special Naval Landing Party commanderOne company Sasebo SNLP (rapid-fire and anti-artillery guns), 15th Establishment Unit Signals Base UnitEstablishment of airfields in area of occupation, secure landing points, repair of roads and bridges
Transport UnitKinryû Maru,
Hakozaki Maru,
Kôtoku MaruTransport

The 25th Air Flotilla had, from the beginning, placed pressure on Allied airbases in eastern New Guinea and to the north of Australia, and had provided air support and reconnaissance for the offensive units.

Landing of the advance party

The invasion force, led by Rear Admiral Matsuyama, left Rabaul as planned at 8 pm on 20 July. It included the main strength of the Yokoyama Advance Party (on board the high-speed Ryôyô Maru and Ayatosan Maru), as well as the naval landing units. The convoy proceeded south-west along the sea lane to the south of New Britain without directly encountering enemy aircraft or submarines. The naval units were successfully debarked 5 kilometres north-west of Buna (Giruwa) at 5.30 pm on 21 July, and the Yokoyama Advance Party at 7 pm in Gona.

The landing was effected with relatively little enemy resistance and continued into the morning of 22 July. The Yokoyama Advance Party had planned to land at Basabua but had mistakenly landed at Gona to the west of the planned position. There were numerous reefs directly off the coast at Buna that did not allow landing craft to approach the shore.

The 25th Air Flotilla conducted special attacks on the airbases around Port Moresby on 18 and 20 July with 27 land-based attack planes, and with 15 and 13 Zero fighters respectively. On 21 and 22 July, they undertook aerial support for the transport convoy. Eighteen Zero fighters were unable to protect the anchorage early in the morning of 22 July owing to bad weather, but did so from 10.45 am to 4.15 pm.[21]

From 6 am that day, approximately one hundred planes attacked the anchorage in six or seven waves. Ayatosan Maru was hit at 7.10 am, resulting in a fire. It was subsequently disabled near the anchorage. The destroyer Uzuki received slight damage from a near miss in aiding Ayatosan Maru and was forced to return to base at Rabaul.

The naval landing party occupied Buna at 7 am on 22 July and began constructing a base. The forward sections of the Yokoyama Advance Party (one infantry company with motor vehicles) moved quickly towards Kokoda as soon as it landed. The main strength of the party proceeded to Buna during the morning of 22 July. It became clear after the landing that there had been a platoon of Australian observer troops at Buna and a smaller party at Gona, but they had withdrawn prior to the Japanese landing.

The 18th Squadron and Uzuki returned to Rabaul along the south of New Britain on 23 July, and the remaining units returned along the north of New Britain by 24 July. Preparations for the second transport proceeded.

The roads around the coast were in relatively good condition. The 40 kilometres of road from Buna and Gona inland to Soputa was passable by motor vehicle without repairs. In other places, movement through the jungle and occasional grassland was difficult for units, with many rivers and steep gullies.[22]

The forward party proceeded quickly from Giruwa, Soputa, and Sonbo along the Kokoda road. At Awala on 23 July, the unit defeated approximately thirty native troops and then one hundred Australian troops. The unit then advanced to the high ground at Oivi approximately 16 kilometres to the east of Kokoda, where they were joined by the main strength of the advance party on 26 July.

Five or six waves of aerial attacks during successive days forced the main strength of the advance party to move at night. They attacked a company of Australian defenders at Kokoda during the night of 28 July, and by the following morning had occupied Kokoda and the adjacent airstrip. The battle took the lives of a company commander and twenty other troops. Australian prisoners of war captured at Kokoda set out the situation of the Australian forces at that time.[23]

Prior to the landing of the Japanese forces, the Australians had only despatched one company to Kokoda and sent observers to Buna and Gona. Upon the Japanese landing, a battalion of Australians in Port Moresby was quickly deployed to Kokoda. This was the 39th Australian Battalion under the command of Colonel Owen. It comprised four companies, A, B, C, and D, with C Company airlifted to Kokoda. The battalion commander, who was in Kokoda, ordered A Company to proceed to Awala to engage the Japanese. Colonel Owen, however, determined that holding Kokoda against a superior Japanese force would be difficult. He consequently withdrew the main strength of the battalion to Deniki with a plan to hold the Japanese in the Owen Stanley Range, leaving one company in Kokoda.

There were approximately five to six hundred native troops led by Australian officers in the area along the banks of the Mambare River. In addition, there were said to be approximately twenty thousand American, Australian, and Indian troops under the command of General Morris in the Port Moresby area, giving a total of around six infantry battalions. The Allied army had established a strong base in the Port Moresby region during the previous six months.

The Yokoyama Advance Party was able to repair roads, making motor transport possible for 60 kilometres from the coast to Sonbo. The road west of Sonbo, however, was suitable only for pack horse, and west of Papaki was only a walking track. The original plan called for advancing and stockpiling munitions and supplies at Kokoda. This stockpile, however, had to be brought back to Sonbo. Each soldier of the advance party was provided with 10.8 litres of rice, or fifteen days’ supplies, to carry from Soputa towards Kokoda.

The 17th Army’s judgment concerning an advance by land

The 17th Army command train left Davao on 22 July and arrived in Rabaul on 24 July to mixed news of the success of the landing of the Yokoyama Advance Party and the abandonment of Ayatosan Maru. The command of the newl formed 8th Fleet also arrived in Truk on 25 July with plans to proceed immediately to Rabaul.

17th Army headquarters, as previously advised by staff officer Tsuji, were expecting imminent notification of the army and navy central agreement concerning the Port Moresby offensive and subjugation operations in eastern New Guinea. However, as previously mentioned, the head of the army Operations Section of Imperial Headquarters on 25 July requested notice of the results of the Ri Operation Study. This was an extremely unforeseen telegram for the 17th Army.

A decision had already been made for the army concerning the overland offensive. In addition, it was determined that after the Buna landing of the Yokoyama Advance Party, the main strength of the South Seas Force would push on the front line using the principle of meeting engagements. Regardless of whether or not the required central agreement had arrived, the landing occurred precisely when it was decided that the main force of the South Seas Force would advance. Both the 4th Fleet and the 8th Fleet were in agreement on this point. Subsequently, on 27 July, transportation switched from the remainder of the Yokoyama Advance Party and the navy’s base force to the transport of the first echelon of the South Seas Force main strength. The 17th Army conveyed to Imperial Headquarters by telegram the events to date along with an appraisal that the overland Port Moresby offensive was, by and large, possible.[24]

Army and navy central agreement

At this point, the army and navy sections of Imperial Headquarters agreed in principle on the execution of an overland offensive to Port Moresby and issued the following army and navy central agreement ("Great army instruction no. 1,318") on 28 July to the army and navy commanders in the field:
Transport of troops to Buna

As previously mentioned, the 17th Army and 4th Fleet (8th Fleet) had changed their plan and decided to transport the main strength of the South Seas Force in the second transport convoy to Buna. However, because the central agreement had not been received, it was decided to revert to the original plan and transport the remainder of the Yokoyama Advance Party and naval base force, as well as military matériel.

Ryôyô Maru and Kôtoku Maru left Rabaul on 27 July under the protection of Tatsuta, Yûzuki, and No. 32 Submarine Chaser. Disembarkation began at Basabua on the evening of 29 July under intense bombardment from Allied planes.

The 25th Air Flotilla had planned to patrol the sky over Buna daily since the landing of the first transport convoy. However, air mobilisation was hampered owing to bad weather, with the exception of patrols by 21 planes on 25 July, and 18 planes the following day.

Direct aerial protection of the convoy on 29 July was entrusted to 27 planes from the navy’s Tainan Air Corps. Approximately ten SBD-3s and four P-39s descended from the clouds at 2.45 pm and attacked the convoy. Kôtoku Maru was hit and took on water before any equipment could be offloaded, though most personnel were rescued. Ryôyô Maru was safe during the day on 29 July, but was attacked at night and was forced to leave the harbour early in the morning of 30 July to return to Rabaul.

The Tainan Air Corps provided air cover again on 30 July with a total of 18 planes. At 7.45 am, five Zero fighters shot down a lone B-17 attempting to attack Ryôyô Maru. Kôtoku Maru, under the protection of Tatsuta and Yûzuki, turned around and headed back for Buna at 1.30 that afternoon, but was engaged by Allied aircraft at around 3.15 pm. It came under attack by eight B-17s at 4.40 pm and was disabled. Tatsuta and Yûzuki rescued survivors from the abandoned vessel and returned to Rabaul on 31 July.

Nankai Maru, under the protection of Tsugaru and No. 28 Submarine Chaser, was especially loaded with part of the 15th Base Establishment Unit and supplies. It left Rabaul on 31 July and headed south, but was attacked by a B-17 en route. As a result, it was forced to abandon its landing and returned to Rabaul on 1 August.

The second transport convoy was completed, but with less than anticipated results owing to the heavy attacks from Allied aircraft.

Imperial Headquarters staff officer Tsuji left for Buna aboard a destroyer on 25 July. He arrived in Rabaul the day after, along with the 17th Army staff. His purpose was to encourage the commander of the Yokoyama Advance Party, who he had known from the Malaya campaign, and to carry out geographical observations on the roads from Buna to Kokoda.

Staff officer Futami (of the 17th Army) described the following incident from that time in his memoirs:
Incidentally, Tsuji was struck by shrapnel on 27 July near the coast of Buna and suffered an injury to his throat. His disembarkation was cancelled and he was repatriated to Rabaul the following day. Tsuji, dissatisfied with the cooperation of the navy’s air corps, immediately contacted the commander of the 25th Air Flotilla and addressed the following message to the Army Department of Imperial Headquarters: "At the end of July, the enemy has air superiority over the Buna area. Although the navy’s air strength amounts to only twenty fighters and thirty bombers, the realities of the situation are not passed on to army command."

Army and navy local agreement

Local agreements between the commanders of the 17th Army, the 8th Fleet, and the 11th Air Fleet were based in the central agreements and went into effect on 28 July.

The fresh command of the 8th Fleet was extremely positive about the operation. First, the 8th Fleet proposed that the navy carry out offensives against Samarai and the coast of Milne Bay near Rabi.[25] As will be discussed below, airfields had previously been constructed by the Allies near Rabi, and by mid-July it acted as a base for an air strength built around P-40s, and had a garrison of approximately five thousand American and Australian troops. However, the Japanese army did not discover the airstrip to the west of Rabi until 3 August. The offensive against Rabi that was discussed in relation to the agreement aimed to secure a midway base for the sea-route operations against Port Moresby via the Coral Sea, and was to be conducted in concert with an invasion and base construction at Samarai. At that time, the presence of the 5,000-strong garrison was completely unknown.

The 17th Army ordered the 35th Infantry Brigade to prepare for operations in the Samarai area, and felt that the Rabi and Samarai operations were the responsibility of the army. Further, the army agreed with navy proposals that the main objective was the invasion of Port Moresby.[26]

Later, however, problems arose between the army and navy concerning the distribution of responsibilities for the proposed sea-route offensive against Port Moresby. The 17th Army considered their responsibilities and proposed the 35th Infantry Brigade for the task. At that time, the 8th Fleet had proposed the following:
The 17th Army asserted that:
The navy, in response, would not change their proposals, stating the opinion that: "Transporting army troops by destroyer will give rise to command-related and other complications. Therefore, we would like the operation to be the responsibility of the navy."[27] The 17th Army determined that they could not hang their hopes on the success of a maritime landing operation, and for the time being decided to leave responsibility with the navy. It was later made clear that the strategic plan of the 8th Fleet would facilitate the operation by landing units at Port Moresby precisely at the moment when it was to fall to the South Seas Force. However, according to the draft of the army and navy central agreement in possession of the navy (the complete text had not yet arrived, and the army had been given the main points in communication from staff officer Tsuji): "In essence, one part of the army unit strength shall be landed near Port Moresby at the appropriate time." In this way, the army stubbornly insisted on taking responsibility for the operation.[28]

Consensus on the agreement was not reached that day. On 28 July, the following day, the navy submitted a further proposal, as follows: "Elements of the army will board navy vessels and be mobilised by sea. In concert with the operations of the South Seas Force, these units will be landed in the vicinity of Port Moresby, thus facilitating the operation."

The 17th Army agreed with this proposal. The army’s wish for these seaborne units to be conveyed by army transport vessels was opposed by the navy because it was felt the speed of these transports and Allied air strength would make it virtually impossible. Consequently, unavoidable limits were placed on the strength of units and the amount of supplies that could be carried.

On the evening of 30 July, the 8th Fleet again discussed landing the army units with the naval landing party, and received consent from the army. The 8th Fleet planned to place the 7th Special Base Force, which was intended for disposition in the Fiji and Solomon Islands area, primarily in the Moresby area.

The main points of the agreement contained in the memorandum drafted on 31 July were as follows:
The first notable feature of this local agreement is the strength of the 17th Army commitment. Whereas the central agreement stipulated a limited strength of six battalions (the South Seas Force and the 35th Infantry Brigade), the 17th Army added the 41st Infantry Regiment to give a force based around a strength of nine battalions. As described earlier, the 17th Army only knew of the conditions in the Port Moresby area after they had occupied Kokoda. It was natural for them to immediately realise the necessity of including the 41st Infantry Regiment as a garrison force.

The second notable feature concerns special provisions for air operations, which included positive planning for not only Buna but for Kokoda. As was made clear earlier, air superiority over the Buna area was firmly in the hands of the Allies, so advancing at least a fighter unit to Buna was an unavoidable demand.

Morale for the Port Moresby offensive among the 17th Army staff, which had arrived in Rabaul on 24 July, was very high. As mentioned, the 17th Army had notified Imperial Headquarters on 25 July their determination that "an offensive against Moresby primarily by the overland route is possible". On 26 July, they telegraphed: "The army has confidence in the success of the overland operation. We beg you not to be concerned."[29]

Operational leadership by the 17th Army

However, the commander of the South Seas Force and staff officers were rather pessimistic about the actual execution of the operation against Port Moresby. Commander Horii of the South Seas Force visited the chief of staff of the 17th Army after the staff office had arrived in Rabaul on 24 July. He stated that it would be impossible to advance beyond Kokoda if he wasn’t given command of all the motor units, not just those attached to the Ryûtô Unit (which was designated on 18 July as the logistics support unit). Further, South Seas Force chief of staff Tanaka stressed that further advance was impossible if 120 tonnes of supplies were not stockpiled at Kokoda. 17th Army staff officer Futami considered these opinions weak-willed and would not act to prevent the deficiencies.[30]

On the other hand, Colonel Yokoyama, commander of the advance party, reported optimistically on 27 July that it would be possible to repair the road between Sonbo and Kokoda, so that if each soldier carried a total of 12 days’ supply – four days to Kokoda and eight days to Port Moresby – then an attack on Port Moresby would be possible at one stretch. The staff of the 17th Army was overjoyed with this report. The judgment that it would take only 12 days to break through from Buna to Port Moresby was based on the knowledge that locals required 12 days to walk this distance.

Commander Horii visited the chief of staff again on 1 August. He stated that previous prisoner-of-war interrogations had revealed that there were approximately twenty thousand Allied troops stationed at Port Moresby. The chief of staff enquired whether the commander had the confidence to defeat an enemy of 20,000. Horii replied that confidence was not the issue, as they had been ordered to mobilise. Rather, he inquired if the strength of the units at the front line would be sufficient, and expressed his disinclination by pointing out the losses they would suffer in the Kumusi River between Buna and Kokoda.[31]

The army chief of staff did not believe that there were 20,000 enemy troops at Port Moresby, and deeply felt that the position of the South Seas Force commander was "grossly overestimating the actual strength of the enemy". However, he cannot be forgiven for completely disregarding the position of the commander. The 17th Army adopted two main measures for the operation.

The first was to summon to Rabaul the 41st Infantry Regiment to strengthen the South Seas Force. The army had initially sent for part of the 35th Infantry Brigade for the sea-route invasion of Moresby when the main strength of army units advanced from Davao. This unit was, however, exchanged for the 41st Infantry Regiment. This measure resulted from orders issued on 1 August. The 41st Infantry Regiment was despatched to Buna and assigned to supply duties, but directly participated in the battle as the necessity arose.

The second measure concerned the point that the conception of the operation, which involved landing the main strength of the South Seas Force at Buna after substantial military supplies had been stockpiled, had changed. Up to that time, the plan was to land the South Seas Force at Buna, provide limited supplies to be carried by each soldier, and make a headlong charge over the mountains to Port Moresby. However, the first and second convoys to Buna had both lost a vessel, and Ryôyô Maru in the second convoy was forced to return to Rabaul suffering engine damage. The planned stockpile was markedly delayed. On the other hand, the Yokoyama Advance Party had continued to make preparations for an airbase at Buna, expecting a favourable development in the local air situation, but did not recognise at the beginning the dangers of the constant air attacks from Allied planes.

At precisely this time, the Army Department of Imperial Headquarters telegraphed the opinion that the timing of the landing of the main strength of the South Seas Force must be determined in the light of the information that the navy planned to strengthen its air presence in the South Pacific, and that the navy air policy was in transition. Consequently, the 17th Army temporarily abandoned its plan to storm through to Port Moresby with no consideration of logistics and supply.

On 3 August, the advance of the main force of the South Seas Force by the third convoy was postponed. In its place, it was decided to accelerate the transport of the navy’s establishment units in order to speed up preparations for construction of an airfield at Buna. If successful, completion of the airstrip was planned for 12 August. The proposal was to strengthen transport of the main force by providing protection from air units advanced to the Buna airstrip.

The 17th Army sent the following telegram to Imperial Headquarters on 6 August concerning their plans up to that time:
The commander of the South Seas Force gathered his senior officers on 7 August, and with the authority of the army chief of staff, advanced planning for the execution of the overland attack on Port Moresby. The senior command of the 17th Army, based on the evidence of the Allied army, which had strengthened preparations for the Port Moresby operation, was concentrating all its strength on this operation.

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Formation of the 8th Fleet and disposition of the units

Plan to change the formation of the fleet in the Pacific area</a>]

The command structure in the Pacific, in the case that operations proceeded according to policies for stage two operations established by the Navy Department of Imperial Headquarters, became an important issue in mid-April 1942. This involved investigation into the proposal to reorganise the fleet in the Pacific after the completion of phase three of stage two (FS Operation) of the Combined Fleet’s operations.

Navy operations to this point in the Central Pacific Area (a sector of command in the South Seas) and the South-East Area (South Pacific) were, of course, conducted with the overall support of the main strength of the Combined Fleet. Air operations were the responsibility of the 11th Air Fleet, with the 4th Fleet responsible for naval surface operations. The surface operations aimed to defend key areas, to patrol and protect shipping, and to seek and destroy enemy ships. The 4th Fleet was therefore an operational fleet, as well as possessing garrison functions owing to the requirements of military administration in occupied territories.

The invasion of key areas in the Fiji and Samoa areas, to be conducted after the Midway Operation, was naturally to be carried out by the main strength of the Combined Fleet. There were, however, several problems to be overcome to undertake grand strategies in the Eastern Pacific Area after the completion of the operations with the military formations described above.

First, it was difficult to unify command over the relatively wide area of responsibility of the 4th Fleet. It would be extremely difficult for one command to coordinate operations to defend key areas, protect shipping, and administer occupied territories, as well as more traditional naval surface operations.

Secondly, it was essential to have a unified command to effectively integrate surface and air operations. Thirdly, a newly established fleet command was desired to allow intimate and direct contact with the recently formed command of the 17th Army.

Subsequent operations in the Central Pacific were to be carried out mainly as air operations. The Navy Department of Imperial Headquarters unified command of air and surface units and considered it vital that an exclusively operational standing be adopted, not one of defence and administration of occupied territories.

As the result of numerous investigations, it was decided to enact a navy structure to reflect this purpose. The proposal to reorganise the fleet in the Pacific area after the completion of phase three of stage two, set for around 18 May, was as follows:

FormationVessels and unitsSpecially installed units
Combined Fleet
4th Fleet4th Base Force(Truk and Parao areas)
5th Special Base Force(Saipan and Guam areas)
6th Base Force(Marshall Islands and Wake Islands areas)
2nd Surface Escort FleetYûbari
29th Destroyer Squadron
30th Destroyer Squadron		Noshiro Maru
Chôun Maru
Attached units4th Harbour Duty Unit
4th Survey Unit
Kunikawa Maru
8th Fleet18th SquadronNatori, Tenryû, Tatsuta
7th Submarine SquadronJingei
21st Submarine Group
22nd Submarine Group
23rd Submarine Group
7th Special Base Force(New Caledonia, Fiji, and Samoa areas)
8th Base Force(New Guinea and Solomon Islands areas)
Attached units10th Establishment Unit
11th Establishment Unit
14th Establishment Unit
Kiyokawa Maru
11th Air Fleet22nd Air Flotilla
24th Air Flotilla
25th Air Flotilla
26th Air Flotilla
2nd Special Base Force(Midway area)
Attached units34th Destroyer SquadronLyon Maru
Keiyô Maru
Nagoya Maru
11th Establishment Unit
12th Establishment Unit
In this way, the duties of the 8th Fleet were basically defined. They included the FS Operation and other surface campaigns in the South-East Area. This also anticipated the establishment of the 7th Special Base Force, charged with garrison duties for the area of the FS Operation under the command of the 8th Fleet. Consequently, the 8th Fleet was not just a garrison force, but an operational fleet under the command of the South Seas Fleet (11th Air Fleet commander) responsible for leading, with substantial reinforcements, surface campaigns in the South Pacific Area.

Change of formation after the naval battle at Midway</a>]

As a result of the battle of Midway on 5 June and the subsequent cancellation of the FS Operation, the above-mentioned planning and research also had to be started afresh. It was clear that it was necessary to reorganise the fleet, which had to that time been devoted to preparations in the South Pacific Area for an invasion of Port Moresby. However, the reorganisation had to be managed with regard to the reorganisation of the Combined Fleet that was happening at that time.

Investigations into these reconstructions were immediately started by Imperial Headquarters and the Combined Fleet. The first issue to be faced followed on from the successive realisation of the progress of naval battles and the overall war situation. The fundamental problem was how the reconstruction should be undertaken to ensure long-term Japanese naval strength to bring the war to a successful end.

The second problem concerned how to reconstruct the Japanese mobile carrier fleet, which for the moment had suffered great damage. Four carriers from the 1st and 2nd Air Flotillas, Akagi, Kaga, Hiryû, and Sôryû, were lost at Midway. There remained, however: Zuihô from the 3rd Air Flotilla (Shôhô had been sunk in the battle of the Coral Sea); Zuikaku and Shôkaku from the 5th Air Flotilla; and Jun’yô and Ryûjô from the 4th Air Flotilla. Repairs on Shôkaku, which was damaged in the Coral Sea battle, were expected to be completed by mid-July. In addition, remodelling of Hiyô, a smaller aircraft carrier, was due to be completed by the end of July.

Until that time, the Japanese mobile carrier fleet had on each occasion been formed from an appropriate strength of battleships, cruisers, and submarines in accordance with the situation. This state of affairs did not arise out of the standing formation, but from the available naval strength. Formalising this arrangement to create a standing fleet would be advantageous from the point of view of training, mobilisation, and command.

Further investigations resulted in reformation of the fleet into a standing mobile fleet. The 1st Air Fleet was stood down and the 5th and 4th Air Flotillas reorganised as the 1st and 2nd Air Flotillas, respectively. With these units as a core, the formations indicated below were reorganised as the newly formed 3rd Fleet:
Orders were issued to form the 3rd Fleet on 14 July, as well as other reorganisations. The commander and chief of staff of the 1st Air Fleet, Vice Admiral Nagumo Chûichi and Rear Admiral Kusaka Ryûnosuke, were retained in these positions for the 3rd Fleet.

Hôshô, Yûnagi, and the 1st Airbase Force (in Kyushu) were also attached to the newly formed 3rd Fleet to be used to train aircrews through a combination of actual battle experience and training exercises. The 3rd Fleet was to be gradually strengthened to the point that the 50th Air Flotilla was expected to be formed by 15 January 1943.

The formation of the 8th Fleet was also ordered to stand alongside the newly assembled 3rd Fleet. The concept of the fleet reorganisation in the Pacific Area, research for which had proceeded as described above, was adapted to incorporate successive essential operations in eastern New Guinea and the Solomon Islands despite the cancellation of the FS Operation. The formation of the 8th Fleet, orders for which were issued on 14 July, was as follows:

Formation and commandUnitsSpecial fleet unitsBase for special fleet units
Combined Fleet
8th Fleet
Chôkai
18th SquadronTenryû
Atsuta
7th Submarine SquadronJingei
13th Submarine Group
21st Submarine Group
7th Base Force23rd Submarine Chaser Squadron
32nd Submarine Chaser Squadron		85th Signals Unit
85th Submarine Base Force
8th Base ForceNo. 20 Minesweeper
No. 21 Minesweeper
21st Submarine Chaser Squadron
31st Submarine Chaser Squadron		5th Gunboat Squadron
56th Submarine Chaser Squadron
81st Garrison
82nd Garrison
84th Garrison
8th Submarine Base Force
8th Signals Unit
Attached unitsTsugaru
30th Destroyer Squadron
(Muzuki, Yayoi, Mochizuki, Uzuki)		Kure 3rd Special Naval Landing Party
12th Establishment Unit		Kure
Kiyokawa Maru
2nd Air Corps
10th Establishment Unit
11th Establishment Unit
15th Establishment Unit		Yokosuka
13th Establishment Unit
14th Establishment Unit
Sasebo 5th Special Naval Landing Party		Sasebo

The 7th Base Force had been newly formed during this reorganisation. Further, the 2nd Air Corps of the attached units was comprised of 16 each of fighters and bombers. This unit was originally intended for the FS Operation. Its orders to form were issued on 1 June, based on the idea that the unit would seize the opportunity to advance to the region after the completion of the invasion of New Caledonia.

The attached units were formed with five establishment units. This provided for an unparalleled capacity in the South Pacific to swiftly construct airbases. In addition, orders were issued between 1 May and 15 June to form other new units. It was planned that the 11th and 12th Establishment Units were to be despatched to Midway, and the 13th and 14th Establishment Units to the area of the FS Operation.

The commander and chief of staff of the 8th Fleet were Vice Admiral Mikawa Gun’ichi and Rear Admiral Ônishi Shinzô. Most of the remaining staff officers were drawn from men who had served at central headquarters in Japan. The senior staff officer was Captain Kami Shigenori, and the next in line was Commander Ômae Toshikazu.

Activation of command of the 8th Fleet</a>]

The Navy Department of Imperial Headquarters issued the following "Great navy instruction no. 112" to the commander of the Combined Fleet:
Furthermore, under this military formation, the commander of the Combined Fleet was also given responsibility for the aforementioned operations with the Outer South Seas Fleet, which included the 8th Fleet strengthened by the 6th Squadron, Yûbari, the 29th Destroyer Squadron, and other units. The 11th Air Fleet was placed in a cooperative role.

The aim of the reorganisation of the fleet in the Pacific Ocean, namely to unify command for surface and air operations, was not, for the moment, realised. The 6th Squadron comprised four ships: Aoba, Kinugasa, Furutaka, and Kako. It had previously been placed under the command of the 4th Fleet and had advanced to Rabaul.

The command group of the 8th Fleet (main strength) left Kure on 19 July on board the flagship Chôkai and arrived at Truk on 25 July. Discussions concerning the succession of command continued during 25–26 July, command being activated at midnight on 27 July.

Staff officers Kami and Ônishi went on ahead to Rabaul, where they occupied themselves with the local agreement with the 17th Army after 28 July. Meanwhile, the standing strength of the 4th Fleet at that time was engaged in the second transport operation to Buna. The command of the 8th Fleet arrived in Rabaul at 3 pm on 30 July and ordered the continuation of current operations based on the old order of battle after moving headquarters from the flagship to land.

The local agreement with the 17th Army was more or less completed on 31 July, but it was amended on 3 August after modifications to the 17th Army’s plans concerning the transport of the main strength of the South Seas Force, as previously discussed. The commander of the 8th Fleet issued the following order of battle on 5 August for the Port Moresby invasion and subjugation of eastern New Guinea operations. This was precisely two days before the Allied landings on Guadalcanal and Tulagi:

FormationCommandStrengthDuties
Main Force8th Fleet commander
Vice Admiral Mikawa Gun’ichi		ChôkaiSupport for the entire operation
Support Group6th Squadron commander
Rear Admiral Gotô Sonchi		6th Squadron (Aoba, Kinugasa, Furutaka, Kako)Support for other units
Escort Fleet18th Squadron commander
Rear Admiral Matsuyama Kôji		18th Squadron (Tenryû, Tatsuta)
Tsugaru, Yûbari
29th Destroyer Squadron (Yûzuki, Yûnagi, Oite, Uzuki)
23rd Submarine Chaser Squadron (No. 22, No. 23, No. 24 Submarine Chasers)
22nd Submarine Chaser Squadron (No. 28, No. 29 Submarine Chasers)
No. 20 Minesweeper
Sasebo Special Naval Landing Party
Kiyokawa Maru observation planes (3)
10th Establishment Unit (part strength)
14th Establishment Unit (part strength)
15th Establishment Unit		1. Escort of Port Moresby invasion units and patrols of anchorage
2. Speedy establishment of Buna area base
3. Occupation of Samarai and speedy establishment of base
4. Mopping up at Cape Ward Hunt
5. Transport of establishment units
Strike Force7th Base Force commander
Rear Admiral Fujita Ruitarô		7th Base Force (missing 23rd, 32nd Submarine Chaser Squadrons)
Seven patrol boats
Kure 3rd Special Naval Landing Party		1. Surface mobile operations
2. Preparations for offensive in eastern New Guinea
Bismarck Area Defence Force8th Base Force commander
Rear Admiral Kanazawa Masao		8th Base Force (No. 20 Minesweeper missing)
Kiyokawa Maru (three reconnaissance seaplanes missing)
Shirataka, Hatsutaka, Wakataka
Futabumi Maru, No. 3 Seki Maru
10th Establishment Unit (part strength)
11th Establishment Unit
12th Establishment Unit
13th Establishment Unit
14th Establishment Unit		1. Landing operations in the Wau area
2. Guard waters near Rabaul
3. Defence of Bismarck area
4. Mopping up and occupation at Madang and Wewak
Submarine Units7th Submarine Squadron commander
Rear Admiral Yoshitomi Setsumi		7th Submarine Squadron (Ro-33, Ro-34)1. Search and surveillance, attack of enemy fleet
2. Destroy enemy surface transport
AttachedKôan Maru plus eight othersSupply

Disposition of various formations</a>]

The disposition of these various formations under the command of the 8th Fleet was as follows:

One company of the Sasebo 5th Special Naval Landing Party and elements of the Base Signals Unit were transported to Buna and occupied the area and the airfield on 21 July 1942. The accompanying elements of the 15th Establishment Unit were responsible for speedily reconstructing the airstrip. The third Buna transport was changed to carry mainly navy establishment units. This was because the transport of the main strength of the South Seas Force had been delayed, and completion of the airfield at Buna was an urgent task at that time. Consequently, the main strength of the 15th Establishment Unit and elements of the 14th Establishment Unit steamed south from Rabaul on 6 August on three transports (Nankai Maru, Kinai Maru, and Kan’yô Maru), escorted by Tatsuta, Yûzuki, Uzuki, No. 23 Submarine Chaser, and No. 30 Submarine Chaser.

The 82nd Garrison and other landing party units, in addition to base force units, were guarding the Lae and Salamaua areas. The 82nd Garrison had been responsible for the defence of Lae since early March (strengthened by the Maizuru 2nd Special Naval Landing Party since 12 April), while one company of the 81st Garrison guarded Salamaua. The only Allied force facing them were elements of an independent force based around the mining town of Wau. The company of the 81st Garrison eventually ceded the defence of Salamaua to a company from the 82nd Garrison, then returned to Rabaul on 28 June 1942.

Meanwhile, a 200-strong enemy force armed with mortars and machine-guns raided Salamaua at around 2 am on 29 June. Japanese casualties amounted to 18 killed and injured. A further 100-strong party raided Lae at 1 am on 1 July resulting in ten Japanese casualties. Two companies of the Sasebo 5th Special Naval Landing Party, which had rushed to Lae, landed under fire from Allied planes on 1 July. These units were placed under the command of the 82nd Garrison and advanced to Salamaua to strengthen its defences.

These counter-attacks by a land-based enemy unit, though small-scale, could not be ignored. Staff officer Yamamoto from the Navy Department of Imperial Headquarters relayed the following information to Imoto from the Army Department on 1 July:[36]

According to the agreement with the 17th Army, the 82nd Garrison was responsible for undertaking diversionary attacks in the Wau area. However, operations to stop the schemes of the Allied units had already been transferred through plans of the 8th Base Force.

Editor’s note: Mopping-up and offensive operations were undertaken from 21–23 July at Nadzab airfield (35 kilometres north-west of Lae) and at Mubo airfield (23 kilometres south-west of Salamaua).

Meanwhile, the airfield on Guadalcanal in the Solomon Islands was completed on 5 August, slightly earlier than planned. This airfield was quickly established for the execution of the FS Operation, which was planned for mid-September and based on the operational leadership policy for the region adopted on 13 June by the Navy Department of Imperial Headquarters, immediately after the naval battle at Midway. Thereafter, the Combined Fleet expected to quickly establish airbases in the South Pacific region, the so-called SN (base establishment) Operation.

Airfield at Guadalcanal

Local navy units had undertaken a search for the most appropriate location for an airbase having previously realised the necessity of a base in the Solomon Islands. A large flying boat from the 25th Air Corps was specially despatched to Tulagi on 25 May. Specially chosen officers and engineers from the 8th Base Force, as well as members of the 25th Air Corps, subsequently undertook a search for a location on Guadalcanal.[37] An appropriate location for a base was found approximately 2 kilometres from the sea to the east of the Lunga River in the north-west of the island.

The commander of the 25th Air Corps wrote to the commander of the 11th Air Fleet on 1 June requesting a survey of the area and the speedy establishment of an airbase:[38]

This request was telegraphed to the chief of staff of the Combined Fleet and the head of the 1st (Operations) Department of the Navy General Staff. It also complied with the overall plans of Imperial Headquarters after the defeat at Midway.

On 19 June, senior officers from the 4th Fleet, 25th Air Flotilla, and the 8th Base Force once again accompanied reconnaissance for a suitable airbase on Guadalcanal. Consequently, the 13th Establishment Unit (Lieutenant Commander Okamura Toku and 1,350 men) and the 11th Establishment Unit (Captain Monzen Tei and 1,221 men) began landing on Guadalcanal on 6 July. An advance party had landed on 1 July, but owing to fears from attack by large Allied aircraft, the main strength of the establishment units was transported in successive groups two vessels at a time. In this way, the landing was successfully completed.

The construction of the airfield commenced on 16 July. Work proceeded quickly despite Allied air attacks. Construction of stage one (an 800 by 60 metre runway) was completed on 5 August.

Lieutenant Commander Okamura had reported the estimated completion date previously, so fighter units were able to advance to the base almost immediately after its completion. Most of the establishment unit personnel were, naturally, construction troops. The number of men combat ready, armed with rifles and pistols, only totalled about one hundred and eighty and one hundred for the 11th and 13th Establishment Units, respectively.[39]

Tulagi and Gavutu

Elements of the Kure 3rd Special Naval Landing Party had occupied Tulagi and Gavutu, two small islands off the southern coast of Florida Island facing Guadalcanal, from 3 May 1942, thus providing a base for the Yokohama Air Corps and small vessel elements. These were strengthened by a company from the 81st Garrison on 1 July, re-forming as the 84th Garrison.

The unit was led by Commander Suzuki Masaaki. The main strength of the unit (approximately two hundred men) were garrisoned at Tulagi, with approximately fifty men on Gavutu, and a detachment of approximately one hundred and fifty men sent to guard the area around Lunga Point on Guadalcanal. In addition, elements of the 14th Establishment Unit had been sent to Tulagi to construct the base.

Surface units and the 2nd Air Corps</a>]

Tatsuta, Uzuki, and Yûzuki had steamed south from Rabaul to escort the third transport convoy to Buna, as mentioned above. Separate surface units, Tenryû, Yûbari, Yûnagi, and others, were at anchorage in Rabaul harbour. The 30th Destroyer Squadron was under the direct control of the 8th Fleet. Its ships, with the exception of Uzuki, were returning to Japan for repairs.

The 13th Submarine Group (submarines I-121, I-122, I-123), under the command of the 7th Submarine Squadron, had completed repairs in Japan and were currently returning to Truk and Rabaul. Submarine combat strength at that time comprised only Ro-33 and Ro-34 of the 21st Submarine Group, while the flagship Jingei was at Truk. These two submarines had been allocated to escort the Port Moresby invasion force so there were no vessels to undertake patrols in the Solomon Islands area. It was planned for the 13th Submarine Group to be advanced to the Espiritu Santo area after they arrived.

Meanwhile, the 2nd Air Corps, the pride of the 8th Fleet, advanced to Lakunai airfield near Rabaul on 6 August and was placed under the command of the 25th Air Flotilla. It was planned for the 2nd Air Corps to advance to Moresby after the 8th Fleet had invaded.

Assessment of the situation

On reflection, how did the command of the newly formed 8th Fleet assess the situation at that time? Reference was, of course, made to this issue in the above-mentioned discussions with the command of the 4th Fleet at Truk.

The 4th Fleet was, as a general rule, leaning towards the rather optimistic opinion that the Allies were probably not considering mounting a fundamental counter-offensive at that time. By way of contrast, the 8th Fleet thought it necessary to expect a counter-offensive sooner rather than later. The target of this counter-offensive was thought to be either the Marshall Islands or Solomon Islands, with the general opinion that the latter was more likely.[41] The 8th Fleet, however, was not concerned enough about a possible counter-offensive to immediately strengthen fighting capabilities.

Of most concern to the commanders of the 8th Fleet was the insufficient strength of the garrisons on Tulagi and at the airfield at Guadalcanal, some eleven hundred kilometres distant from Rabaul. The 8th Fleet recognised the need to advance fighter plane units to Guadalcanal immediately on the completion of the airfield. Proposals to this effect were strongly pressed to the staff officer responsible for the 11th Air Fleet, Captain Takahashi Senjun, who was at that time visiting Rabaul.[42] Captain Takahashi had come to Rabaul to discuss the Port Moresby operation, and to investigate the feasibility of using the airfield on Guadalcanal. This followed an extremely agitated appraisal concerning the despatch of fighters to the airfield, which was telegraphed from Lieutenant Commander Okamura, the head of the 13th Establishment Unit stationed at that time on Guadalcanal.

However, the 11th Air Fleet had made no preparations to quickly advance fighter units to Guadalcanal. Even the 8th Fleet had no plans to advance its own 2nd Air Corps to Guadalcanal, as they had previously been earmarked for deployment to Port Moresby.

Allied raids against Tulagi and Guadalcanal intensified from about the time the command of the 8th Fleet arrived in Rabaul. Previous raids to that time had consisted of only one to three planes on reconnaissance or offensive raids. The raids suddenly intensified after 31 July, with daily attacks by seven to ten aircraft. This situation is recorded by the overview reports of the 25th Air Flotilla, as follows:

In the New Guinea area, meanwhile, a new airfield was discovered at the beginning of August near Rabi in Milne Bay. A large number of aircraft were confirmed to be stationed at the airfield. It was also clear that the region had been strongly reinforced by Allied transport ships. The difficulties encountered during the transport operations to Buna have been previously discussed.

Command of the 8th Fleet judged from these conditions that the Allies’ main focus would be to stop the Japanese offensive against Port Moresby, and that attacks on Guadalcanal would be limited to raiding operations.[43]

The locals on Guadalcanal fled into the mountains on 5 August. However, the Japanese forces thought this was the result of local conditions. No one imagined this was a forewarning of a full-scale Allied invasion.

Despite this, the 8th Signals Unit attached to the 8th Base Force noticed a change in signals communications towards the end of July and judged that the Allies were planning some strategy. This intelligence was passed on to the appropriate commanders. The 8th Fleet chief of staff, Rear Admiral Ônishi, recorded in his memoirs that he personally visited the 8th Signals Unit and listened to these communications. He states that these indicated an imminent Allied counter-offensive, and that the target would be Guadalcanal. Despite this, the 8th Fleet did not undertake discussion of any mechanisms to meet this threat.

The Navy Department of Imperial Headquarters was also aware of the change in Allied signals. The head of the 1st (Operations) Department despatched the following telegraph to the chiefs of staff of all fleets on 4 August:

Further, the Navy Department received intelligence in mid-July that "A large transport convoy (37 ships) left port from San Diego on 2 July, and another from somewhere on the west coast (45 ships), both expected to reach the waters to the east of Australia by early August."

In regard to this information, the Navy Department judged that: "The Allied objective is simply to reinforce and support Australia. Given Japanese inability to advance to Australia, it is thought that the Allies will instead land reinforcements directly at Port Moresby."[44] The chief of staff of the Combined Fleet, Vice Admiral Ugaki, directly alerted the navy chief of staff, Admiral Nagano, of this intelligence on 17 July.

However, even the Combined Fleet did not discuss any particular strategies based on this intelligence to counter the threat. It was anticipated that the formation of the 8th Fleet and the unprecedented preparations undertaken in the South Pacific Area were the result of these considerations.

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Operations of the 25th Air Flotilla

The air war of attrition intensifies

As discussed above, the outbreak of the battle of the Coral Sea resulted in orders issued on 10 May postponing the invasion of Port Moresby. The 25th Air Flotilla subsequently returned to its previous duties.

These previous duties involved patrols of the region, pursuit and destruction of enemy offensive units, destruction of Allied strength in the north-east of Australia, and cooperation with the Port Moresby offensive operation (based on orders from the Airbase Force Command issued on 10 April). At this stage, the operational policy of the 25th Air Flotilla was outlined as follows:

The order of battle was essentially unchanged, but the base of operations had changed somewhat. The main strength of the land-based attack units (4th Air Corps and the Motoyama Air Corps) were assembled at Rabaul (Vunakanau). The main strength of the fighter unit (Tainan Air Corps) had been advanced to Lae, with elements stationed at Rabaul (Lakunai). Elements of the Yokohama Air Corps (the main strength from 3–17 May) were advanced to Tulagi.

The fighting strength in the area on 15 May was as follows:

It goes without saying that the 25th Air Flotilla faced a difficult air war of attrition. The policy was adopted to mainly use the fighters for the offensive air war and to hold the strength of the attack planes in reserve.

The Rabaul detachment of the 24th Air Flotilla (15 Zero fighters) was temporarily placed under the command of the 25th Air Flotilla on 25 May 1942. It was further strengthened by 12 float planes from Japan on 3 June, and by three Type-96 land reconnaissance planes despatched from the 3rd Air Corps on 5 June.
The Japanese force operated raids from bases in Rabaul and Lae between mid-May and mid-June primarily against Port Moresby, and occasionally against Horn Island. The Allies flew raids in large aircraft against Rabaul and in medium aircraft against Lae. This was surely an air war of attrition.

The Allies mainly bombed Rabaul with B-17s, and Lae with B-25s and B-26s. As a rule, the Allied air raids increased in intensity when Japanese raids on Rabaul subsided. Although the Allies bombed Lae, this was mostly undertaken unaccompanied by fighters. The Japanese Zero fighters were overwhelmingly superior to Allied fighters, but they were ineffective against Allied bombers, particularly the B-17, which the Allies proudly called the "flying fortress". For example, six B-17s raided Rabaul at 1.15 pm on 25 May from an altitude of 8,000 metres. Fifteen Zero fighters took-off, six of which engaged the B-17s. One Allied bomber in particular was pursued and attacked for two hours, suffering repeated attacks. One engine was damaged, but the Zeros could not force it down.

Attacks on Port Moresby by lone fighters were discontinued after mid-June owing to the offensive operations. These were replaced by a combination of night attacks by small groups of fighters, and daytime raids using fighters and bombers in combination. The number of operational aircraft at that time did not exceed thirty, with fighters outnumbering bombers. There were only three night raids and ten daytime raids (including one against Horn Island) during the period from mid-June to mid-July.

Several missions to reconnoitre roads for the Ri Operation Study were undertaken by land-based reconnaissance and attack aircraft under escort by fighters during the period from late June to early July. The reports from these missions were introduced above as evidence in support of the overland invasion route.

A decision was taken in mid-July to despatch the first transport of the Yokoyama Advance Party to Buna for the Ri Operation Study. The commander of the 25th Air Flotilla issued the following operational policy on 17 July to enable cooperation between the 18th Squadron and the South Seas Force:[45]

The campaign was executed according to these general principles after the second transport to Buna.

The night raids at this time on key locations along the eastern coast of Australia by Type-2 flying boats are worthy of mention. The Airbase Force Command issued orders to the command of the 25th Air Flotilla on 17 July titled, "Orders for the reconnaissance of the Fiji, New Hebrides, and New Caledonia areas and the execution of night raids on key locations in eastern Australia by Type-2 flying boats". Two Type-2 large flying boats from the 14th Air Corps Detachment of the 24th Air Flotilla were placed under the command of the 25th Air Flotilla especially for this purpose. These flying boats advanced to Rabaul on 20 July, raided Townsville during the nights of 25 and 28 July, and Cairns during the night of 31 July. Although the targets on each occasion were airfield facilities, the success of the raids was not clear.

While the Japanese forces had difficulty dealing with the might of Allied B-17s, the new B-24 made its first appearance in the South Pacific Area in July. The main strength of the 11th and 13th Establishment Units landed on Guadalcanal on 6 July. At 2.40 pm that day, they were raided by one large aircraft. It was pursued by six float planes, but regrettably escaped. This was a B-24. It was reported that its speed was approximately 20 kilometres per hour superior to that of the float planes. However, there were reports that two B-24s were shot down: one of the two from the raid on 10 July, and one on 17 July. After those incidents, B-17s were the aircraft used to raid Guadalcanal.[46]

Airbase preparations

The airbase war after that ebbed and flowed, arriving at a situation where both sides held their own. The remarkable change up to early August was the steady preparation of new Allied airfields in the Port Moresby area. At that time, the Japanese had only two airfields at Rabaul: Vunakanau and Lakunai. Even counting the main base for the 25th Air Flotilla at Lae, this totalled only three airfields. (The strip at Gasmata was only for emergency landings.) From the end of June, the Combined Fleet began the SN Operation, a ground-breaking attempt to quickly establish airbases in the South Pacific Area. The unit mainly responsible for this operation was the 4th Fleet, but the 25th Air Flotilla was charged with reconnoitring suitable sites for bases, and responsible for protecting establishment units while under transport and while constructing bases.

The bases at Guadalcanal and Buna were swiftly constructed in accordance with the terms of the SN Operation. Elsewhere, the airfields at Rabaul and Lae were repaired and enlarged, and an airfield constructed at Kavieng. The problem area was in the central-north Solomon Islands. The distance from Guadalcanal to Rabaul is about 1,000 kilometres, so there was no question that a land base was required between the two. Reconnaissance of Kieta and Buka islands was undertaken by a single aircraft on 19 June. A runway was discovered around 6 kilometres from the southern coastline of Kieta (800 by 80 metres), and a further runway in the south of Buka Island (700 by 60 metres). The latter had an emergency runway (500 by 60 metres) arranged in an "L" formation with the main runway.

The staff officers responsible for the 25th Air Flotilla and the 8th Base Force, and the commander of the 14th Establishment Unit, personally flew over and inspected the Kieta runway in a large flying boat on 8 July. As a result of this reconnaissance, the commander of the 25th Air Flotilla sent a report outlining his conclusions to the Airbase Force Command that evening, as follows:[47]
This telegraph was also sent to the Combined Fleet and the Navy Department of Imperial Headquarters. The Airbase Force Command received this message and submitted the following request with regard to the Combined Fleet the next day (9 July):[48]

The plans to construct the base on Kieta were, regrettably, discarded, a decision with which the Navy Department agreed. On 26 July, a special detachment was sent to personally inspect the Buka base, but this too was considered unacceptable as one of the speedily constructed bases.

Reconnaissance of the situation of the Allies

The 25th Air Flotilla, in addition to reconnaissance over north-eastern Australia for its own operations, carried out reconnaissance from time to time over the Santa Cruz Islands, the New Hebrides, New Caledonia, and the Fijian islands area.

Reconnaissance of the New Hebrides and the Chesterfield Islands (in north-east New Caledonia) was carried out on 1 June by flying boat from the Tulagi base. Special orders had been issued by the Airbase Force Command for the reconnaissance, at the request of the 2nd Fleet. The mission confirmed that the Allies had been steadily constructing an airbase on Efate in the New Hebrides. Tulagi is approximately 1,100 kilometres from Rabaul, and 1,300 kilometres from Efate. It will be recalled that in planning for the FS Operation after the battle of Midway, consideration was given for an invasion of Efate as a stepping stone for an invasion of New Caledonia.

The Airbase Force Command, on 22 June, demanded reconnaissance as soon as possible of the Santa Cruz Islands and the New Hebrides areas for suitable sites for flying boat and land airbases. Subsequently, while undertaking patrols over patrol sector F, one aircraft secretly reconnoitred Efate and another surveyed the New Hebrides and Santa Cruz. The latter of these missions was accompanied by Imperial Headquarters staff officer Major Yamauchi Tomiaki, who had been at that time sent to the South-East Area to gather intelligence.

Airbase Force Command specially transferred the 14th Air Corps Detachment to the 25th Air Flotilla on 17 July.[49] Reconnaissance to this point had mainly looked for suitable sites for airbases to be used for the New Caledonia invasion. From this point on, however, its role would be strategic reconnaissance to uncover Allied movements. The commander of the 25th Air Flotilla, after the completion of the night raids on key sites in northern Australia, ordered the 14th Air Corps Detachment on 2 August, as follows: "Two Type-2 flying boats will quickly advance to Tulagi, carry out reconnaissance according to oral instructions from 3 August, then return to Rabaul." The two Type-2 flying boats carried out these orders and undertook strategic reconnaissance over Fiji and New Caledonia on 4 August.

Meanwhile, an American composite force under the command of Rear Admiral Richmond Turner and based on the 1st Marine Division had completed exercises on Koro island in Fiji, and after departing on 31 July, was steaming towards Guadalcanal with the escort fleet. The convoy passed to the south of the New Hebrides during 3 August, then turned to the north-west at a position to the north of New Caledonia the following day (latitude 159 degrees 6 minutes east, longitude 16 degrees 36 minutes south). This was the huge invasion fleet group comprising some 82 vessels.[50]

Unfortunately for the Japanese forces, the two Type-2 reconnaissance flying boats were unable to locate the invasion group. The commander of the 25th Air Flotilla filed the following initial report: "Two Type-2 flying boats carried out reconnaissance over the Fiji and New Caledonia areas and returned successfully to Tulagi. Report of intelligence to follow."

On reflection, patrol duties had been one of the 25th Air Flotilla’s main strategic responsibilities. Patrols had continued on a daily basis after 25 April.

Three flying boats set out on 4 August to patrol sector F(a). Poor weather, however, limited no. 1 and no. 2 reconnaissance aircraft to a patrol radius of 120 kilometres, and no. 3 aircraft to a range of 740 kilometres. Visibility was barely 9 kilometres. Other than passing a large enemy aircraft at a bearing of 170 degrees and 180 kilometres from Tulagi, there was nothing to report.

The same three aircraft patrolled sector F(a) again on 5 August. Again, the patrol was hampered by heavy rain and limited to a radius of 180 kilometres. No sign of the enemy was found. On 6 August, no. 1 reconnaissance plane patrolled an area of 690 kilometres, and no. 2 and no. 3 each patrolled an area of 740 kilometres. The convoy carrying the US marine division was within the range of these patrols, but were not caught within the trap. A thorough search of the sector was completed, with the report of "Scattered clouds, occasional squalls, visibility from 20–40 kilometres, enemy not sighted."[51] A huge opportunity had been lost.

The Rabi airfield

To return somewhat to earlier events, the 8th Fleet had decided on the basis of investigations carried out by the army and navy at the end of July to invade key areas in Milne Bay and Samarai. The commander of the 25th Air Flotilla consequently ordered the 4th Air Corps on 1 August to undertake aerial photographic surveillance of Samarai, which was carried out on 3 August by a Type-1 land-based attack plane from patrol sector A. During this mission, however, what looked like an airfield was discovered to the west of Rabi (approximately 55 kilometres north-west of Samarai), with two small aircraft about 500 metres above the airfield. The reconnaissance aircraft was sent again to investigate the following day, 4 August, with an escort of four Zero fighters. An air battle ensued and the land-based attack plane in the end did not return. The 25th Air Flotilla compiled the results of intelligence and submitted the following report:[52]

The Allies had recognised the need for an airfield in Milne Bay in May, and thereafter began construction of the base for use by fighter units. Instructions issued on 12 June expanded the base for use by bombers. By mid-July, the base had an Australian infantry brigade of around four thousand five hundred men, as well as a US fighter battalion equipped with P-40s and anti-aircraft defence units.

In any case, the appearance of the airfield at Rabi was a great threat to the Japanese forces that had first planned to invade Port Moresby. Without first taking this airfield, it would be impossible to mount an attack on Port Moresby by the sea route. Even if the overland offensive against Port Moresby was successful, it was not possible to ignore the airfield at Rabi.

The commander of the 25th Air Flotilla initially planned to attack the Rabi airfield with his entire force. Orders were issued for all Zero fighters that were combat ready to leave Lae and assemble at Rabaul. The following day at 11.30 am, the entire strength of the Tainan Air Corps (including 12 aircraft providing direct cover) and 27 land-based attack planes from the 4th Air Corps were ordered to attack the Rabi airfield and destroy all enemy aircraft in the area. As it transpired, the American invasion force was at that time in the waters to the east of Rennell Island heading north towards the west coast of Guadalcanal.

The number of operational aircraft available to the 25th Air Flotilla on 6 August was barely as follows:
On that very day, the fresh 2nd Air Corps under the command of the 8th Fleet arrived in Rabaul and were incorporated into the 25th Air Flotilla. The operational aircraft of the 2nd Air Corps numbered 15 Zero fighters and 16 carrier-based bombers.

Plan to transfer the 26th Air Flotilla to the South Pacific Area

During these events, the 11th Air Fleet was planning to advance the 26th Air Flotilla to the South Pacific Area and have it replace the 25th Air Flotilla.

The 26th Air Flotilla was formed at the same time as the 25th Air Flotilla, as previously mentioned. It comprised the 6th Air Corps (based on fighters), the Mizawa Air Corps (land-based attack planes), and the Kisarazu Air Corps (land-based attack planes). The standing strength of the formation, including reserves, was sixty fighter planes for the 6th Air Corps and 36 land-based attack planes for each of the Mizawa and Kisarazu Air Corps.

The 26th Air Flotilla was stationed primarily in the Mizawa and Kisarazu areas of Japan at the beginning of stage two operations, and was responsible for operations in the home islands and in the North-East Pacific Area. At the time of the Midway Operation, the 24th and 26th Air Flotillas were advanced to the Midway area to participate in later offensives against Hawaii.[53]

Consequently, the fighter units of the 6th Air Corps were aboard the mobile carrier fleet and directly participated in the Midway naval battle. Further, elements of the Mizawa Air Corps had been advanced to Wake Island and made responsible for reconnaissance attack operations.

After the Midway battle, the Combined Fleet continued preparations for the execution of the FS Operation using units from the 2nd Fleet, the 1st Air Fleet, and the 11th Air Fleet.

The staff office of the 26th Air Flotilla received on 30 June an explanation of the overview of the FS Operation that was to commence on 20 September.[54] The content of this, which had been arranged with the 11th Air Fleet, was that the unit would advance to Guadalcanal on around 20 August, when stage two of the airfield construction was completed. Then, under protection from units at the airfield, they would first attack Efate, then New Caledonia. For this purpose, the 26th Air Flotilla was strengthened with units that included the Yokohama Air Corps (flying boats), the 14th Air Corps (flying boats), and the 2nd Air Corps (fighters and bombers). With Guadalcanal and Tulagi as bases, Type-2 flying boats were to attack New Caledonia, and land-based attack aircraft to attack and harass Efate. The 2nd and 6th Air Corps were to advance to Efate when it had been occupied, from where they could attack and harass New Caledonia.

Meanwhile, the plan for airfield construction on Guadalcanal comprised two stages. Stage one involved accommodating 27 fighters and 27 land-based attack planes, while stage two would aim to cater for 45 Zero fighters and 60 land-based attack planes. In other words, the advance of the main strength of the 26th Air Flotilla to Guadalcanal was planned for early September. Until then, units would remain in their current disposition.

However, the FS Operation was cancelled in July, so the plan was changed such that the 26th Air Flotilla would replace the 25th Air Flotilla and be responsible for air operations in the South Pacific Area.

To this end, the staff officers of the 11th Air Fleet and the 26th Air Flotilla met on 1 August. The 11th Air Fleet proposed that elements of the 6th Air Corps be advanced to the newly completed airfield on Guadalcanal as soon as possible after its planned completion date on 5 August. Meanwhile, seven float planes from the Yokohama Air Corps were patrolling the skies over Tulagi. However, it was considered necessary to transfer these to protect the transport convoy operations to Buna.[55] The 6th Air Corps had participated in the Midway battle and lost a large portion of its complement of aircraft, as well as suffering attrition to its aircrew. At this time, the unit was being rebuilt at Kisarazu. It was estimated that from 9–12 aircraft could be loaded on an aircraft carrier and advanced to Guadalcanal by around 16 August.

Further, a proposal was drawn up concerning the advance of the main force of the 26th Air Flotilla into the South Pacific Area. The airbase at Kavieng, built as part of the base establishment policy, would be completed by 15 August to the extent that it could accommodate the main strength of the 6th Air Corps and the Mizawa Air Corps. The base would be expanded so that, in addition to the Mizawa Air Corps, which would arrive in the latter part of August, the air flotilla headquarters and the remainder of the 6th Air Corps would arrive in early September. The Mizawa Air Corps was at that time stationed in Saipan. It was planned that the Kisarazu Air Corps would advance to the area in late September or early October. The transfer with the 25th Air Flotilla could then happen after that date.

Meanwhile, Lieutenant Commander Okamura from the 13th Establishment Unit had submitted his demands for air strength to be advanced to the Guadalcanal area. In response, the 11th Air Fleet senior staff officer and supply staff officer were despatched to Rabaul to undertake discussions concerning the Port Moresby operation and to undertake firsthand reconnaissance of the Guadalcanal base area. They had planned to set out from Rabaul for Tulagi aboard a Type-2 flying boat on 6 August, but were delayed by one day owing to discussions about the Port Moresby operation with the 17th Army and the 8th Fleet. On the following day, 8 August, the US army landed troops on Guadalcanal.

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Plans of Imperial Headquarters in early August

From the Pacific Ocean to Chungking and the Indian Ocean

The situation of the local army and navy forces in the face of a US counter-attack on Guadalcanal has been described. It is now necessary to turn to an examination of the overall strategic plans of Imperial Headquarters.

Notions of a quick decisive battle on the surface of the Pacific Ocean held by the Army Department and Navy Department of Imperial Headquarters were frustrated in early July after the results of the battle of Midway. The prime concern of strategic leadership at that time shifted to operations against Chungking and the Indian Ocean.

Research into detailed strategies for seizing power in Chungking had proceeded since the time when the operational overview for stage two was determined by the Army Department in mid-March. However, by the latter part of June, the army’s Operations Department had changed its position and was pressing for the No. 50 Operation, an invasion in the first instance of Xian, then continuing into an offensive along the Yangtze River, ultimately leading to No. 51 Operation to invade both Chungking and Chengdu.[56] The campaign was to be the scale of the Szechwan invasion force of 15 infantry division and two air divisions, and was due to commence in December 1942 and be completed in March the following year.

Meanwhile, another issue that emerged at that time, and that flowed on from the end of the Burma campaign, was whether to invade eastern India to foment Indian discontent with Britain and promote civil disorder.

The Burma campaign was basically over by early May. The Southern Area Army consequently despatched its staff officers to Tokyo to present appraisals of the plan to invade India. The army’s Operations Department requested the Southern Area Army to proceed with detailed research into the feasibility of the plan, while waiting for the results of its own investigations.

The Army Department of Imperial Headquarters at that time favoured political stratagems and destruction of merchant shipping rather than a land invasion. The latter was considered too difficult, and there were fears that it would result in indigenous resistance from the general population.[57]

The invasion of Ceylon, however, would be beneficial to the development of German and Italian operations in the Western Atlantic Area. They held the idea that the India operation would be executed if it could be carried out in collusion with Germany and Italy.

A decision had not been reached by 26 June, but the Army Department issued instructions for an outline of the preparations for the Ceylon operation, and ordered the designated infantry group (38th Division) to carry out preparatory training exercises.

Meanwhile, the staff office of the Combined Fleet presented a proposal on 22 June to the Navy Department outlining the commander of the Combined Fleet’s proposal to cancel the FS Operation. The proposal also argued the merits of using the mobile carrier fleet in the Indian Ocean to smash merchant shipping routes in order to force Britain to sue for peace in the area. Later, in early July, the FS Operation was indeed cancelled, and the Navy Department and Combined Fleet proceeded with research into operations in the Indian Ocean area.

There were two aims to the Indian Ocean operations. First was primarily to try and promote separation and civil unrest in India by invading Ceylon and key locations in India such as Calcutta. Secondly, enforcing a blockade between India and Britain by intensifying operations against merchant shipping was an attempt to break British will to continue the war.

The operation against key locations in India and the blockade operations in the Indian Ocean were related. There were pre-existing plans of the army for the former operation, but it was waiting on further research. The navy was conducting its own independent research for the supply-route blockade operation. The focal point of the discussions involved large-scale proposals to send submarines, and also surface units, to the Indian Ocean to do battle with enemy surface units.

The bones of an agreement were hammered out by early August as the result of discussions between the Navy Department and the Combined Fleet. This involved a force based on the 2nd and 3rd Fleets sortieing from Japan on 20 September. Supply preparations and operations in the western Indian Ocean and to the north-west of Australia would be completed in stage one beginning in early October and taking around a month. After that, stage two operations would commence in early November in the Bengal Bay area.[58]

In this way, from late June through to early August, the Navy Department of Imperial Headquarters was undertaking various investigations into three different operational proposals: the Chungking invasion operation, the invasion of key locations in India, and the supply blockade in the Indian Ocean. The Army Department was completely obsessed with realising the plan to invade Chungking. The navy placed most importance on the Indian Ocean operation. Both operations had their merits and demerits. Army minister Tôjô transmitted the following opinion to chief of operations Tanaka on 21 July:
Imperial Headquarters’ operational policy in the South Pacific Area

Let us now bring together the ideas of Imperial Headquarters relating to operations in the South-East Area (South Pacific Area) carried out under the overall operational leadership policy.

First, the "Army and navy central agreement relating to operations in eastern New Guinea" was issued on 28 July. According to this agreement, the operational objective was, "To invade and secure the key areas of Port Moresby, to annihilate the enemy from eastern New Guinea, and in combination with taking advantage of the Solomon Islands, to bring the Coral Sea under control." Originally, this agreement primarily specified the invasion of Port Moresby, but it did not specify any tactics for after the invasion, especially in the Solomon Islands region. However, the clause "in combination with taking advantage of the Solomon Islands, to bring the Coral Sea under control" provides just a glimpse of the notion of adopting a defensive stance on the important eastern New Guinea and Solomon Islands fronts concerning the Coral Sea.

However, the operational leadership policy for subsequent operations adopted by the Navy Department of Imperial Headquarters in early July made clear the following:
The policy to strengthen counter-offensives against Allied operations to reclaim key areas was announced. Further, the navy chief of staff, in his submission to the emperor dated 11 July, stated:

This makes clear that the intention of the strengthening of the Japanese strategic position was to try and smash the Allied counter-offensive at the line of eastern New Guinea, the Solomon Islands, and the Gilbert Islands.

The beginning of the Combined Fleet’s SN Operation in late June to swiftly establish airbases in the South Pacific was a manifestation of the conception of this strategic position.

Army chief of staff Sugiyama also pointed out on 11 July that:
This prompted caution in assistant chief of staff Tanabe and operations chief Tanaka.[59]

As these examples make clear, Imperial Headquarters shifted for the moment from an offensive to a strategic defensive position with regard to the South Pacific Area. It was also the reality that they anticipated a fast Allied counter-attack in the direction of this region. However, it is also true that no specific discussions or research was carried out jointly by the Army and Navy Departments concerning the operations to break this counter-attack, particularly defensive operations in the Solomon Islands area.

Even at the Guadalcanal airfield, which was originally established to support the invasion of New Caledonia, there were few defensive mechanisms installed. Furthermore, senior officers in the Army Department and staff officers had not even heard of Guadalcanal, let alone that the navy had established an airfield there.[60] The army held no interest and disregarded any information on Guadalcanal despite the navy having made clear their intentions.

Of final concern is Imperial Headquarters’ intention for local army and navy units to "Pursue investigations and gather intelligence relating to operations against mainland Australia and the invasion of New Caledonia and Fiji areas."[61] This was made clear in the above-mentioned submission to the emperor on 11 July by the army chief of staff. At that time, the FS Operation had not been completely called off. Consequently, Imperial Headquarters originally established a strategic defensive policy, but did not pay sufficient attention to the establishment of their line of defence. The urgent task in the area was thought to be the speedy occupation of Port Moresby, so their strength was wholeheartedly concentrated on achieving this goal.

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Notes

1 Futami Akisaburô, Kodôki oyobi kaisôroku (Record of a beating heart and recollections) (1967).
2 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
3 Dairikumei, Daikairei, Dairikushi, oyobi Daikaishi no kakutsuzuri (Bundles of Great Army Orders, Great Navy Orders, Great Army Instructions, and Great Navy Instructions).
4 Sakusen kankei jûyô shorui tsuzuri (Important documents related to operations).
5 Kawaguchi Kiyotake, Gadarudanaru Shim ani okeru Kawaguchi Shitai no sakusen (Operations of the Kawaguchi Attachment on Guadalcanal) (1954).
6 Toyofuku Tetsuo, Toyofuku Shôsa kôwa (Interview with Major Toyofuku) (1943).
7 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
8 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
9 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
10 Toyofuku Tetsuo, Toyofuku Shôsa kôwa (Interview with Major Toyofuku) (1943).
11 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
12 Kanezawa Masao, Kanezawa nisshi (Diary of Kanezawa).
13 Dai 18 Sentai sentô shôhô (Detailed battle reports of the 18th Squadron).
14 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
15 Futami Akisaburô, Kodôki oyobi kaisôroku (Record of a beating heart and recollections) (1967).
16 Futami Akisaburô, Kodôki oyobi kaisôroku (Record of a beating heart and recollections) (1967); and Matsumoto Takeshi, Sakamoto Takeshi no kaisô (The memoir of Matsumoto Takeshi).
17 Futami Akisaburô, Kodôki oyobi kaisôroku (Record of a beating heart and recollections) (1967); and Matsumoto Takeshi, Sakamoto Takeshi no kaisô (The memoir of Matsumoto Takeshi).
18 Matsumoto Takeshi, Sakamoto Takeshi no kaisô (Recollections of Matsumoto Takeshi).
19 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
20 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
21 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
22 Toyofuku Tetsuo, Toyofuku Shôsa kôwa (Interview with Major Toyofuku Tetsuo).
23 Toyofuku Tetsuo, Toyofuku Shôsa kôwa (Interview with Major Toyofuku Tetsuo).
24 Minami Taiheiyô sakusen shiryô (Documents of operations in the South Pacific).
25 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
26 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
27 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
28 Konuma Haruo, Gashima ni okeru Dai 17 Gun no sakusen (Operations of the 17th Army on Guadalcanal) (1957).
29 Minami Taiheiyô sakusen shiryô (Documents of operations in the South Pacific).
30 Futami Akisaburô, Kodôki oyobi kaisôroku (Record of a beating heart and recollections) (1967).
31 Futami Akisaburô, Kodôki oyobi kaisôroku (Record of a beating heart and recollections) (1967).
32 Kaigun senji hensei tsuzuri (Navy wartime organisation).
33 Kaigun senji hensei tsuzuri (Navy wartime organisation).
34 Sakusen kenkyû shiryô (Operations research documents).
35 Sakusen kenkyû shiryô (Operations research documents).
36 Imoto Kumao, Imoto Kumao Chûsa nisshi (Diary of Lieutenant Colonel Imoto Kumao).
37 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
38 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
39 Okamura Tokunaga, Okamura Tokunaga Shôsa no kaisô (Recollections of Lieutenant Commander Okamura Tokunaga).
40 Sakusen kenkyû shiryô (Operations research documents).
41 Ômae Toshikazu, Ômae Toshikazu Chûsa no kaisô (Recollections of Commander Ômae Toshikazu).
42 Ômae Toshikazu, Ômae Toshikazu Chûsa no kaisô (Recollections of Commander Ômae Toshikazu).
43 Ômae Toshikazu, Ômae Toshikazu Chûsa no kaisô (Recollections of Commander Ômae Toshikazu).
44 Sanagi Kowashi, Sanagi Kowashi Chûsa nisshi (Diary of Lieutenant Colonel Sanagi Kowashi).
45 Dai 25 Kôkû Sentai sentô shôhô (Detailed battle reports of the 25th Air Flotilla).
46 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
47 Dai 25 Kôkû Sentai sentô shôhô (Detailed battle reports of the 25th Air Flotilla).
48 Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
49 Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
50 Frank Hough, Pearl Harbor to Guadalcanal, pp. 252–254.
51 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
52 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
53 Shibata Funzô, Shibata Funzô Chûsa no kaisô (Recollections of Commander Shibata Funzô).
54 Shibata Funzô, Shibata Funzô Chûsa no kaisô (Recollections of Commander Shibata Funzô).
55 Shibata Funzô, Shibata Funzô Chûsa no kaisô (Recollections of Commander Shibata Funzô).
56 Imoto Kumao, Imoto Kumao Chûsa nisshi (Diary of Lieutenant Colonel Imoto Kumao).
57 Tanaka Shin’ichi, Tanaka Shin’ichi Shôshô no nisshi ni motozuku shuki (Notes based on the diary of Major General Tanaka Shin’ichi).
58 Sanagi Kowashi, Sanagi Kowashi Chûsa nisshi (Diary of Lieutenant Colonel Sanagi Kowashi).
59 Tanaka Shin’ichi, Tanaka Shin’ichi Shôshô no nisshi ni motozuku shuki (Notes based on the diary of Major General Tanaka Shin’ichi).
60 Tanaka Shin’ichi, Tanaka Shin’ichi Shôshô no nisshi ni motozuku shuki (Notes based on the diary of Major General Tanaka Shin’ichi).
61 Sakusen kankei jûyô shorui tsuzuri (Important documents related to operations).


Translated by: Dr Steven Bullard

Original text: Bôeichô Bôei Kenshûjo Senshishitsu (ed), Senshi sôsho: Minami Taiheiyô Rikugun sakusen <1> Pôto Moresubi–Gashima shoko sakusen (War history series: South Pacific area army operations (1), Port Moresby–Guadalcanal first campaigns) (Tokyo: Asagumo Shinbunsha, 1968): 167–230.
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