Australia–Japan Research Project at the Australian War Memorial

Army operations in the South Pacific area: Papua campaigns, 1942–1943
Chapter 2: The advance to the Solomon Islands and eastern New Guinea

(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): 53–118.)
Translated by Dr Steven Bullard


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Chapter contents

Chapter 2: The advance to the Solomon Islands and eastern New Guinea

Operational plan by Imperial Headquarters


Offensive operations in Lae and Salamaua
Occupation of strategic areas in the northern Solomon Islands and Admiralty Islands by the 4th Fleet
The air war of attrition
Sea-route invasion of Port Moresby
Outline of the battle of the Coral Sea
Notes
Operational plan by Imperial Headquarters

As described previously, it was evident from 24 January 1942, the first day after landing, that the Japanese offensive operations against Rabaul and surrounding key areas were not being met by strong Allied resistance. Two airfields and the good natural harbour that protected the strategic operational area of Rabaul had fallen into the hands of the Japanese army, in addition to the outer defensive base at Kavieng.

The Yokohama Air Corps, which had advanced to Rabaul immediately after the offensive, conducted reconnaissance over Port Moresby on 24 January. They reported no more than one special duty ship and four small vessels at harbour, and only two large and one small aircraft at the airfields to the east of the township.[1]

Rabaul was bombed by the Allies three times after 9 pm on 24 January, resulting in damage to one army transport ship.[2] Night bombing raids on Rabaul were conducted in small parties by the Allies every other night thereafter, heralding the beginning of the air war of attrition between Rabaul and Port Moresby that would escalate over time. It was also a prelude for the bitter fighting that would continue over the following three years.

Offensive operations by the Japanese army against key locations in the southern area developed more quickly than anticipated, even while the bitter fighting against American and Filipino troops on the Bataan Peninsula dragged on into mid-January. Japanese forces had advanced through the Malayan peninsula, and reached the Johore Strait by the end of January. By this time the operational army in the Netherlands East Indies had placed pressure on the main island of Java through positions on Borneo, the Celebes, and the Moluccas. The army’s invasion of the Tenasserim district airbase in Burma was completed with the occupation of the strategic location of Moulmein on 31 January.

In response to this situation, Imperial Headquarters undertook to advance operations for the southern area by one month, and decided to speed up the completion of the entire Burma area operations. Orders to this effect were issued on 22 January.

Orders from Imperial Headquarters and the central agreement between the army and navy

The Army and Navy Departments of Imperial Headquarters commenced investigation into operations to be conducted after the completion of the opening offensives (stage one operations, Southern Area key offensive operations) from early in 1942.[3] The results of these investigations were incrementally adopted and implemented independently by Imperial Headquarters, or through discussion in Imperial Headquarters–government liaison committees. This process will be examined in detail in the next chapter. This chapter will discuss operations adopted by Imperial Headquarters from the end of January to early February to invade key areas in the Solomon Islands and eastern New Guinea – newly devised operations that were not included in planning at the beginning of the war.

Editor’s note: War leadership after the completion of the Southern Area key offensive operations was established with the "Outline of essential future war leadership" dated 7 March 1942.

The Navy Department of Imperial Headquarters issued "Great navy instruction no. 47" on 29 January, while the Army Department issued "Great army instruction no. 596" on 2 February (unofficially telegraphed on 28 January). This order was promulgated as follows:
The army and navy central agreement on which the commanders of the South Seas Force and the Combined Fleet based the execution of this operation is cited below:[4]

The continuation of offensive operations into Tulagi in the southern Solomon Islands, and into Port Moresby on the south coast of eastern New Guinea was an expansion of the offensive scope of planning that had been established by Imperial Headquarters at the beginning of the war. Only a fraction of the entire force of the army and navy was committed to these operations, even though the objective was roughly 1,000 square kilometres from Rabaul. Moreover, the orders were adopted prior to conclusions emerging from overall war leadership, or even from operational leadership.

What was the purpose of these operations? Were they an attempt to incorporate the Port Moresby and Tulagi areas within the so-called "final line of offence" (opposed to the "line of defence") of the South Pacific region; or to promote a base for direct operations against the Australian mainland, or islands in the Fiji and Samoa areas? These questions will be addressed in a following chapter.

The operational policy section of the army and navy central agreement included the indefinite conditions that "the navy will seek an opportunity to independently (or in cooperation with the army according to the conditions) invade Tulagi …", and "the army and navy will cooperate after the completion of the invasions of Lae and Salamaua to invade Port Moresby". These consultative decisions were entrusted entirely to the field commands of the army and navy in the region.

This operation was set in motion by a great order (provisionally called a "Great army order") issued by the Army Department of Imperial Headquarters, and by instructions (provisionally called "Great navy instructions") issued by the chief of staff of the Navy Department of Imperial Headquarters to the command of the Combined Fleet.

The navy considered this operation to fall within the scope of the fundamental responsibilities granted to the commander of the Combined Fleet at the start of the war ("Great navy order no. 9"). Consequently it was mobilised by the navy chief of staff simply according to this army and navy central agreement.

Editor’s note: The main operational responsibilities granted to the command of the Combined Fleet based on "Great navy order no. 9" were as follows:

Offensive operations in Lae and Salamaua

Military preparedness before the operations

A flying boat from the Yokohama Air Corps discovered an airfield at Surumi, approximately 300 kilometres south-west of Rabaul, on 27 January 1942. There were suspicions that the Allies would use this as a relay base for attacks on Rabaul. Detailed air reconnaissance confirmed the existence of a paved airstrip and five Western-style buildings. Further, approximately twenty buildings were discovered at Gasmata, 5 kilometres from Surumi. However, there were neither defensive installations nor an airstrip at this site.

Section 3 of orders to the South Seas Fleet, which came into effect on 1 February, outlined the responsibilities for units in the "R" area, as follows: "The Surumi area will be invaded as soon as possible and an airbase established." This was postponed, however, because of air raids from a US task force in the Marshall Islands on 1 February. The commander of the 4th Fleet telegraphed that it would be "possible for the Surumi operation to commence after 8 February" and ordered the command of the 8th Special Base Force to undertake a limited offensive.

An outline of the operational plan was as follows:[5] The invasion force and support group sortied from Rabaul between 5.15 am and 6.22 am on 8 February. The heavy rain at that time provided cover for the fleet. Landings were made the following day at 4.40 am at Surumi, and at 5.00 am at Gasmata. There were no Australian troops to oppose either landing.

Munitions and supplies were immediately debarked, airfields were quickly established, and defensive preparations for the area commenced. The support vessels for the invasion force were slightly damaged during aerial attacks by small numbers of planes on 10 and 11 February.

Approximately one hundred and seventy soldiers from the naval landing force were installed as a garrison force at Surumi on 13 February, while the main strength returned to Rabaul. The formation of the Surumi Invasion Force was dissolved on 15 February.


The airfield at Surumi was long enough to service carrier-based fighters (800 metres by 100 metres) and was operational by 12 February.[6] The airfield had great value as a relay base for eastern New Guinea and the Lae and Salamaua areas.[7]

By the end of February 1942, the situation at other airfields was as follows:

LakunaiNo hindrance to use by fighters[8]
VunakanauUseable by land-based bombers; being repaired and widened[9]
Kavieng1,200 metres by 90 metres unpaved strip; difficult for land-based attack planes after rain[10]
The 24th Air Flotilla, which provided the main air strength in the region, was called the South Seas Fleet Airbase Western Area Attack Force. Its strength at the beginning of March was as follows:
3. 1st Air Corps (placed under the command of the 24th Air Flotilla from the 21st Air Flotilla): 18 Type-96 land-based attack planes. The 1st Air Corps also had 9 planes at Tarawa (in the Marshall Islands).[11]

There were no changes to either the South Seas Force of the army after it returned to garrison duties from 5 February, or to the navy’s 8th Special Base Force after the offensive against Surumi.

The army and navy held a combined Empire Day banquet on 11 February hosted by the command of the 8th Special Base Force. The commander, Rear Admiral Kanazawa, recorded in his diary for that day: "Banquet. Army officers were surprised at our Western-style food. Collaboration between the army and navy is most satisfactory." One that same day, the troops of the 25th Army in Malaya entered the streets of Singapore.

The disposition of the Allied forces near Lae and Salamaua

Fighter planes were operational from the airstrips at both Lae and Salamaua. Each strip was 800 to 1,000 metres long and 100 metres wide.

There was also a small airfield at Wau, approximately 50 kilometres south-west of Salamaua. Expansion of airfields and various preparations were also continuing at Port Moresby.

Allied airbase units continued to attack Rabaul and the Gasmata region from bases in Townsville and Darwin, using Port Moresby as a relay base and Lae, Salamaua, and Wau as forward bases.

Lae and Salamaua each were staffed by approximately fifty to one hundred volunteer troops – civilians who gathered to receive military training three or four days in the week. Aerial photographs and forward reconnaissance by commanders of the invasion force revealed only what looked like a small, scattered base. There were, of course, no capital ships or aircraft stationed there.

Meanwhile, the movements of the American task force that raided the Marshall Islands on 1 February were unclear. Units were consequently on high alert owing to the high chance of further raids in the Bismarck Archipelago region.[12]

The army and navy local agreement and orders to the South Seas Force

The terms of the army and navy local agreement concerning the invasion of the Lae and Salamaua areas were agreed in principle on 13 February between the chief of staff of the South Seas Force, Lieutenant Colonel Tanaka Toyonari, who had been despatched to Truk, and the command of the 4th Fleet.

The agreement, based on this understanding, was finally established on 16 February. The main points were as follows:

At noon on 17 February, the commander of the South Seas Force issued orders to Major Horie Masao, the commander of the unit directly responsible for the offensive, the 2nd Battalion of the 144th Infantry Regiment:[13]
Major Horie undertook reconnaissance of the terrain and enemy situation in the Salamaua area in a navy aircraft on 19 February, two days after receiving these South Seas Force orders. Commanders of the various units subsequently gathered at headquarters on 20 February and received operational preparation orders concerning unit disposition and transport details.

On precisely that day, the navy discovered a US aircraft carrier task force moving north-west approximately 740 kilometres north-east of Rabaul. As will be described later, this force sortied against and attacked air and surface units of the 4th Fleet. This delayed the planned landing, with discussions held on 25 February deciding on a new landing date of 8 March.[14]

Preparations for the offensive by the 4th Fleet

The attack by the US carrier task force resulted in no discernible advantage. Contact was broken on 21 February, so a golden opportunity was lost. Consequently, it was decided to continue with preparations for the invasion of Lae and Salamaua. The following orders were telegraphed to individual units on 25 February:

South Seas Fleet Operational Telegraph Orders No. 109[15]
The strength of dispositions indicated at this time was as follows:

CommandStrengthMain responsibilities
4th Fleet (Vice Admiral Inoue Shigemi)
Lae, Salamaua Area Invasion Force6th Torpedo Squadron (Rear Admiral Kajioka Sadamichi)(see attachment below)1. To attack and destroy the enemy in the Lae and Salamaua area in cooperation with the army.
2. To secure this area, quickly establish an airbase, and cooperate with air operation units.
Support Group6th Squadron (Rear Admiral Gotô Sonshi)6th Squadron (Aoba, Kako, Kinugasa, Furutaka)
18th Squadron (Tenryû, Tatsuta)
23rd Destroyer Squadron (Kikuzuki, Uzuki)		Support for Lae–Salamaua operation
Air Unit24th Air Flotilla (Rear Admiral Gotô Eiji)4th Air Corps (main strength Yokohama Air Corps)
1st Air Corps, Kamoi (Editor’s note: submarine tender)		Assist Lae–Salamaua operation
Bismarck Area Defence Force8th Special Base Force (Rear Admiral Kanazawa Masao)(abbreviated)Assist Lae–Salamaua operation

The operational outline for each unit of the 4th Fleet indicated in the above disposition chart was determined as follows:
Battle orders to the Horie Battalion

The following battalion order, based on South Seas Force orders issued on 17 February, was transmitted by Major Horie at 1 pm on 28 February:

There are three distinctive features of these battalion orders:

1. Two operational outlines based on different conceptions were conveyed to the various units in the diagrams contained in section two of the orders.

2. Section three of the orders (unit responsibilities) distributed strategic responsibility according to only one of these conceptions.

3. The organisation of vessels and movement at sea for both plans was the same. (Consequently, the formation of army units was the same, but the responsibilities differed according to the proposal.)

The prevailing winds at that time were strong and from the north-east. It was unknown whether it would be possible to land at the coastline to the south of Salamaua on the short route between the airfield and township, or to land at the coastline to the west. Eventually, the battalion commander, after long and careful consideration of the options, judged that the former option had better odds for success.

The operational outline, based on historical records, was as follows:
Detailed agreement with the escort fleet

The landing of the 16th Army on Java began on 1 March 1942. On that day, the South Seas Force reached an agreement at Rabaul with the 6th Torpedo Squadron, which provided a direct escort for the force. Chief of staff Tanaka, battalion commander Major Horie Masao, and the escort fleet chief of staff, Commander Enokio, met to thrash out the details. The commanders of both forces approved the draft agreement reached the following day, 2 March.[16]

The details of the agreement were virtually the same as for the Rabaul operation. However, there were two proposals for the route: to the south of New Britain (proposal A), or to the north (proposal B). Distinctive to these proposals were the different departure times – at 5 pm and 5 am on 5 March respectively – and the fact that the movement of the army units after landing was in part established and mostly agreed on, as follows:
3. After mopping-up operations in the designated areas has been completed, the force will as soon as possible hand the garrison over to the Tomita Unit of the navy and return to Rabaul.[17]

Agreement on the problematic issues of the escort at sea and protection of the landing point was reached as follows:
2. Protection from the air (and submarine) will be conducted during day s–1 by two fighters from the 24th Air Flotilla and two reconnaissance planes from Kiyokawa Maru. Protection of the landing point will be the responsibility of the 29th Destroyer Squadron, the 14th Minesweeper Flotilla, and Kiyokawa Maru.[18]

The escort fleet, as mentioned previously, was comprised according to section 3 of offensive orders of the South Seas Force. However, in contrast to the army component of two transports, the navy had provided two light cruisers, six destroyers, and one special seaplane tender (in addition to three navy transports).

The offensive operation

The Horie Battalion boarded Yokohama Maru and China Maru on 3 March, and was fitted out according to the army and navy agreement the following day. It was decided on 4 March to adopt proposal A for the sea-route, namely the course to the south of New Britain. Comprehensive training was undertaken on both 3 and 4 March. All units of the army and navy invasion force and support group were assembled in Rabaul by the morning of 5 March. From that time, a lone Lockheed flew in the skies over Rabaul.[19]

The 24th Air Flotilla stepped up its aerial attacks and reconnaissance over Port Moresby, Lae, and Bulolo (approximately 20 kilometres north of Wau) and elsewhere each day from 3–7 March. What looked like large-style aircraft were discovered at Salamaua and Wau.[20]

The invasion force sortied from Rabaul at 1 pm on 5 March; the support group followed at 4 pm. Aircraft aboard Kiyokawa Maru maintained daily reconnaissance of the enemy and conducted aerial patrols. Fighter units from the 24th Air Flotilla carried out direct protection in the skies over the invasion force. The army and navy units divided at 9 pm on 7 March for Lae and Salamaua respectively. Successive tropical storms hit from 7 March, limiting visibility and making it a difficult task to enter the anchorage.[21]

The army transport fleet encountered a violent storm at 10.30 pm when it entered the anchorage area to the east of Salamaua. The wind at that time was 20 kilometres per hour from the south-west, with the seas 1.8 metres on the coast. The landing barges were lowered at 11 pm, with boarding completed by midnight. They headed for the south-east coast of Salamaua at 12.15 am.

Despite several signal flares being visible over the land around the time the fleet entered the anchorage position, the Horie Battalion reached the landing point unopposed at the planned time of 12.55 am. One aircraft left the ground just at that moment. The advance began immediately and the airfield was successfully occupied by 3 am.[22]

The Australian troops had in fact withdrawn to the south during the night of 7 March, leaving none of their units in the Salamaua and Kela areas. Consequently, battalion commander Horie altered his plans. He despatched elements of his force to occupy the Salamaua township and Kela, and kept his main strength assembled near the airfield.[23]

By 4.30 am, the Salamaua township and Kela were successfully occupied. Combined intelligence reports indicated that the Australians had withdrawn to Wau along the San Francisco River valley, and the local people had evacuated to Wau and Port Moresby in the face of the Japanese army’s invasion of Rabaul and advance to this area. Wau was the centre of the gold mining district and was reached by a narrow road from Salamaua.

After dawn, a lone Lockheed bombed the transport ships three times, resulting in slight damage to Yokohama Maru, with three killed and eight casualties. Lieutenant Colonel Tanaka, the chief of staff of the South Seas Force aboard the flagship Yûbari, telegraphed the commander of the necessity of controlling the air with navy air units owing to increasing concerns over Allied air attacks.[24]

The Kure Special Naval Landing Force troops landed at the coast to the south of Lae at 2.30 am on 8 March and occupied the airfield and township unopposed.

The establishment units began work on the airfields during the morning of 8 March, with preparations for its use as a fighter base completed by 1 pm the following day.[25] Mopping-up operations were completed in the area during the day of 9 March, with two high-angle anti-aircraft guns debarked and placed in position during that afternoon. A single Lockheed also attacked Lae, but the only damage was a single strike on the destroyer Asanagi. The original text mistakenly names this destroyer "Asakaze".

The support group entered Queen Carola Harbour on the west side of Buka Island during the morning of 9 March. In addition to minesweeping operations, a naval landing force landed on the island and carried out mopping-up on the coast around the anchorage the following day.[26]

Allied air raids after the landing

Although Lae and Salamaua had been occupied virtually without incident, it was not long before there were repercussions. Lae and Salamaua were attacked at 7.40 am on 10 March by numerous carrier-based fighters and large-scale bombers. Torpedo bombing and strafing attacks were carried out against units on land and at sea. The attacks continued in waves until 9.45 am, with approximately twenty torpedo bombers, forty carrier-based bombers, eight Lockheeds and eight B-17s. The entire Japanese strength was mobilised immediately, from the patrol planes aboard Kiyokawa Maru to the naval and land units, resulting in the reported shooting down of around ten Allied planes.[27] At that time, fighter units from the 4th Air Corps had planned to advance to the area following the completion of the airfield preparations at Lae. Poor communications, however, resulted in 11 fighters missing the battle, arriving in the area from 1 to 3 pm.[28]

The damage to Japanese forces from this attack was as follows:

This was the first time the Allies had inflicted large-scale damage during a counter-attack since the start of the war, and was a portent of the future direction of campaigns in the South Pacific Area. However, stage one of the Southern Area operations was successfully completed ahead of schedule following the unconditional surrender of the Netherlands East Indies on 9 March. The issues raised by the successful counter-attack were hidden in the shadow of these brilliant successes.

These Allied surprise air attacks will be discussed in detail in a later chapter.

Five B-17s and one Lockheed attacked Lae on 11 March. The Japanese fighter squadron intercepted and downed the Lockheed, but the poor condition of the runway resulted in damage to two fighters.

The Horie Battalion, engaged in the offensive at Salamaua, handed over the garrison to the naval landing force at noon on 12 March in accord with the previously mentioned agreement. The entire unit left Salamaua aboard China Maru at 2 am on 13 March, and arrived in Rabaul two days later. The seaborne invasion force, with the exception of the naval landing force and establishment unit, continued to provide escort and returned to Rabaul at the same time.[29]

The support group at Queen Carola Harbour departed for Rabaul at 4 pm on 10 March. However, it sortied again from Rabaul at 5 pm on 14 March and returned to Queen Carola Harbour at 7 am the following day to patrol in preparation for engagements with the US task force. The support group was transferred to Kavieng on 17 March, thus bringing to completion the offensive against Lae and Salamaua, the first planned advance by the Japanese army into New Guinea proper.

Occupation of strategic areas in the northern Solomon Islands and Admiralty Islands by the 4th Fleet

Occupation of strategic areas in the northern Solomon Islands

4th Fleet Headquarters wanted to invade and position the 8th Special Base Force in key areas in the northern Solomon Islands as soon as possible after the completion of the invasion of Lae and Salamaua.

The target locations were Shortland Island and Kieta on the north coast of Bougainville. Shortland Island was positioned at the south-eastern end of Bougainville. The so-called "Shortland anchorage" comprised the area between these two islands and Ballale Island. This area permitted a large force to shelter, with the inlet at the eastern side of Shortland Island suitable as a seaplane base.

The airfield at Kieta and the area between the coast and the nearby islands were promising locations for a navy base. Both could act as transit bases for the offensive against Tulagi, the ultimate objective in the Solomon Islands area. Precautions at that time were only taken against the movements of the US task force, as the existence of large numbers of ground troops was not anticipated.

Rear Admiral Kanazawa of the 8th Special Base Force, the commander of the invasion force, took direct command of the 30th Destroyer Squadron. He boarded the flagship destroyer Muzuki and sortied from Rabaul at 10 am on 28 March.

On board was a two platoon-strong naval landing force. Protection was provided by the 6th and 18th Squadrons. The naval landing force disembarked at Shortland Island after Shortland anchorage was entered at 2 am on 30 March. No enemy troops were located on the ground, and only one enemy aircraft flew overhead at 10 am. One platoon from the naval landing force was deployed to establish the seaplane base.

The invasion force left Shortland anchorage to the north at 4pm on 30 March, and occupied Kieta at first light the following day. There was absolutely no resistance at this location either. Command of the invasion force withdrew the naval landing force troops, which arrived in Rabaul at 11.30 am on 1 April. The operation was completed.

Occupation of strategic areas in the Admiralty Islands</a>]

The Admiralty Islands consist of Manus Island and other isolated islands in the area. It was a rear strategic location situated deep into the lateral area of the region. There was, as previously mentioned, a small airstrip at Lorengau on the north-east of Manus Island. The so-called "Lorengau anchorage" between Manus Island and the crescent-shaped Los Negros Island could accommodate large fleet vessels. The northern half of Los Negros was flat ground covered in coconut palms suitable for an excellent airfield.

The 4th Fleet had planned to send the 6th Torpedo Squadron to invade the Admiralty Islands immediately after the offensive at Rabaul. However, it was decided to invade Gasmata first, so the Admiralty operation was, for a time, set aside.

For the first stage of the operation, an invasion force sortied from Rabaul on 6 April, occupying the Lorengau airfield and anchorage on 8 April. On that same day, the north-western New Britain mopping-up force entered the bay at Talasea (eastern coast of the Willaumez Peninsula in central-north New Britain), landed unopposed and proceeded to clean up the area.

The 8th Special Base Force was reorganised on 10 April. Its name was changed to 8th Base Force, and the naval landing force was attached as the 81st and 82nd Garrisons.

The air war of attrition

The air war that centred over Rabaul from early February to April 1942 was more ferocious than any within the sphere of the Pacific War at that time. Further, it was a substantial problem from the standpoint of the overall direction of war strategy. Allied ground troops had offered no resistance during the invasion operations against key areas in eastern New Guinea and the Solomon Islands, so the advance proceeded as planned. However, the situation had developed such that it was increasingly difficult to ignore the Allied counter-offensive from airbase units and a carrier task force. The situation surrounding the air war of attrition at Rabaul at this time had direct consequences on later ground operations in the region. Consequently, several aspects of the actual situation will be discussed in detail here.

Destructive air raids on Port Moresby in early February

The circumstances surrounding the 24th Air Flotilla’s advance for the Rabaul offensive has previously been discussed. Allied air raids conducted by lone or small numbers of aircraft on Rabaul occurred every other night from 24 January until 3 February. The aircraft were Short Sunderland flying boats of the Royal Australian Air Force operating from a base at Port Moresby. The air raids generally came during the early evening, at 8 or 9 pm. A lone reconnaissance plane was also occasionally seen during the day. These raids prompted return fire from the 8-centimetre high-angle anti-aircraft guns of the 8th Special Base Force, or from guns on vessels at anchorage in the harbour. Carrier-based reconnaissance planes from Kiyokawa Maru were also deployed. However poor communication with the searchlight units meant that neither method was particularly successful as the planes could not be located. [31]

The first air raid on Port Moresby by the Yokohama Air Corps, which was delayed by poor weather conditions, was finally conducted on 3 February. Eight Type-97 flying boats split into two formations and set out during the night, bombing the township of Port Moresby from 1.30 am to 2.30 am on 3 February. All aircraft returned safely to Rabaul.[32]

A further air raid on Port Moresby was carried out the following night by five large flying boats. With this focus on Port Moresby, Allied air raids on Rabaul completely ceased for a time, although Gasmata received daily air raids after Japanese forces occupied the area on 9 February.

Editor’s note: According to the US Air Force history, the air strength at Port Moresby at this time was: two squadrons (small number of aircraft) of Catalina flying boats; one squadron of Hudsons; and one squadron of Wirraways.

Strategic background and manoeuvres of the US carrier task force</a>]

After suffering heavy losses at Hawaii at the opening of the war, a strategic review of the options open to the US task force left no room for a defensive position in its fundamental strategy against Japan. Prior to the outbreak of war, the US defensive strategy for the Pacific had centred on the Hawaiian islands. From Hawaii the US fleet could secure the triangular maritime region between Midway Island, Johnston Island, and Palmyra Island, and protect the United States–Australia supply line between the US mainland and the 180 degree meridian, while also including the islands of Fiji and Samoa.

This idea did not fundamentally change even after the appointment of Admirals King and Nimitz to the US navy high command at the end of December 1941. Admiral King, commander-in-chief of the US Fleet, immediately appointed Admiral Nimitz, commander-in-chief of the Pacific Fleet, to the task of securing this triangular region, as well as maintaining the supply line between the United States and Australia, including the islands of Fiji and Samoa.

At the same time, however, and also to appease public opinion, the US navy carried out strategic attacks against the previously mentioned Japanese bases and maritime transport fleet using submarines and high-speed aircraft carriers. Furthermore, there also was an optimistic opinion that this would check the advance of the main strength of the Japanese forces into key areas in the southern region. The attack on the Marshall Islands on 1 February, and the successive air raids by the US task force on Rabaul (20 February), Wake Island (24 February), Marcus Island (4 March), Lae and Salamaua (10 March), and Tokyo (18 April), were based on this strategic policy.

The Japanese advance to Rabaul on 23 January caused great anxiety to the government of Australia. On 27 January, the government warned Admiral Nimitz, the commander-in-chief of the Pacific Fleet, that there were reasonable fears that the Japanese would attack Noumea in New Caledonia prior to its occupation by US forces, at that time en route to the area. The Australian government held concerns that they were exposed to a Japanese advance to Port Moresby. On the other hand, the Americans felt that the Japanese would advance into the New Hebrides and New Caledonia areas after they had gained a secure foothold in Rabaul.

Given these conditions, Admiral King ordered Admiral Nimitz to despatch the carrier task force (based on USS Lexington), under the command of Vice Admiral Brown, to the Fiji and New Caledonia area and to place it under the command Vice Admiral Leary of the ANZAC Area.

Rabaul air raid from the Lexington Task Force</a>]

The US carrier group that had attacked the Marshall Islands on 1 February consisted of a task force under the command of Rear Admiral Fletcher built around the aircraft carrier Yorktown, and a second group based on the carrier Enterprise under the command of Vice Admiral Halsey. The total strength of the force was two aircraft carriers, three cruisers, and four destroyers.

In response to the incursion by the US task force, the 24th Air Flotilla at Rabaul instigated daily patrols of the waters to the north of the Solomon Islands and the Coral Sea from 5 February using two to four flying boats.[35]

According to postwar investigations, the force that attacked the Marshall Islands had returned to Hawaii at the time of the heightened patrols by the 24th Air Flotilla, with a different force drawing near to Rabaul after having set out from Hawaii on 31 January.

This was the above-mentioned task force led by Vice Admiral Brown (hereafter called the Lexington Task Force), with a strength of four heavy cruisers and ten destroyers in addition to Lexington. The sole objective for the offensive planned by Brown was none other than the Japanese-occupied Rabaul. This recommendation was readily approved by Brown’s superior, Vice Admiral Leary.[36]

The Lexington Task Force proceeded south-west and swept the waters to the north of Fiji and Samoa after passing through the seas to the east of the Phoenix Islands on 8 February. The Japanese forces received intelligence that "an enemy carrier force was proceeding to the Samoa area" on 7 February, but it thereafter disappeared.[37]

The task force veered to the north, while east of the New Hebrides, on 15 February and proceeded north-west through the waters to the east and north of the Solomon Islands. It was Vice Admiral Brown’s plan to despatch bombers from a position 230 kilometres from Rabaul at 2 am on 21 February.

However, the Japanese forces received intelligence that two enemy destroyers had suddenly appeared in the waters near Truk on 19 February. This report proved to be false, but flying boats were despatched from Rabaul during the evening of 20 February to carry out reconnaissance for the enemy destroyers in the rear waters near Truk. The lead plane radioed the following message at 8.30 am before losing communications: "Large enemy force 110 kilometres north-east of Rabaul, heading north-east."[38] The 4th Air Corps lifted from Vunakanau airfield at 12.20 am after receiving orders from 24th Air Flotilla command to engage the enemy force.

As the 4th Air Corps was not at full strength at that time, it was unable to put fighters in the air to support the assault group (bomber group), and the bomber’s torpedoes had not even arrived. However, the aircrews had participated in the superb campaigns in the South-West Area at the beginning of the war and were flushed with these successes. They had great confidence in the outcome of the assault using only fixed bombs.

The Lexington Task Force, however, proceeded north observing extremely strict radio silence. At 8.50 am on 20 February, several unidentified aircraft were identified by radar on board Lexington (these were the first fitted in the US navy and could not show altitude). These aircraft were the above-mentioned patrol flying boats.

The assault group that had sortied from Rabaul reported the discovery of the enemy carrier force at 2.35 pm and again at 3 pm. The Rabaul base waited expectantly for a report indicating a successful attack.

However, two bombers in the first wave of nine were brought down by US fighters before they reached their targets. The other seven were able to drop their bombs, but the carriers took evasive action and were able to avoid any direct hits. Fighters launched from the carriers engaged with the bombers in the skies over the task force. Without an escort of fighters, all in the first wave of Japanese land-based bombers were destroyed. Similarly, three bombers in the second wave of eight were brought down prior to reaching the target, and two more wre brought down immediately after bombing the carrier force.

During the attack, two Japanese bombers damaged by US hits attempted unsuccessfully to ram the carriers. Of the remaining three bombers, only one returned to the Rabaul base, with one ditching into the ocean and the other crash-landing in Simpson Harbour.

The Americans lost one pilot and two fighters. Commander Brown, however, fearing the opportunity to bomb Rabaul was lost, abandoned the planned air raid and withdrew without further action.

The Japanese had lost 14 land-based bombers, two flying boats, and one reconnaissance seaplane, with the two ditched bombers suffering serious damage. Although the damages were great, the planned air raid on Rabaul had been averted.

Air raid on Salamaua</a>]

Vice Admiral Brown, in the light of the failed air raid on Rabaul, submitted the following proposals to Pacific Fleet Commander Nimitz:
Nimitz accepted both requests and assigned him Yorktown under Rear Admiral Fletcher, thus forming a mobile force based on two aircraft carriers.

The Lexington Task Force, which had withdrawn to the north-east on 20 February, veered to the west in the waters to the east of Santa Cruz Islands and entered the Coral Sea. It joined the Yorktown group to the south-west of Espiritu Santo soon after it had entered the patrol region of Japanese reconnaissance planes. At that time, there were no suitable targets for air raids other than Rabaul.

It was precisely at this time that Japanese forces had landed in the Lae and Salamaua area, presenting a perfect target for air attack by the task force. The problem was whether to take the dangerous course through the Bismarck Sea to launch an assault, or to launch an attack over the 5,000 metre-high Owen Stanley Range from the waters to the south of eastern New Guinea. Suitable nautical charts of the Bismarck Sea for the first option had not been prepared and, furthermore, this would take the carrier force too close to Rabaul.

The task force launched an assault group from 80 kilometres off the south coast of eastern New Guinea on 10 March, some 900 kilometres from Rabaul on a line through Lae and Salamaua. By 6.40 am, 104 planes were airborne.

The air raid was a success, with Japanese losses as previously described. The Americans lost only one aircraft and one pilot. (Editor’s note: This differs from the Japanese record of losses.)

The following is taken from the detailed battle report on Tsugaru, which had participated as an escort in this battle:
Air battles over the bases from late February to early March

Allied air attacks, which as previously mentioned had temporarily ceased, began again on 23 February. Early that morning, five American four-engine B-17s surprised Japanese troops in a daring attack. This was the first appearance of the B-17, a plane the Japanese were thoroughly unable to contend with in the South-East Area from that point on.

Thereafter, small scale air raids against Rabaul continued uninterrupted. The trend was for a lone reconnaissance plane during the day to be followed by an attack from small numbers of bombers at night. There were also, however, numerous occasions when B-17s would attack during the day, especially early in the morning. The raids were generally directed towards the airfields and ships at anchorage in the harbour. Damage to aircraft and personnel was insignificant but steadily mounted.

Japanese anti-aircraft measures were at that time inadequate to cope with this level of air attack.

As the majority of the land-based attack planes planned for Rabaul had been destroyed in the raid on the US carrier task force on 20 February, a squadron of land-based attack planes from the 24th Air Flotilla, which were training on Tinian, were quickly advanced to Rabaul. The total air strength in the South-East Area was further strengthened when the Combined Fleet diverted a detachment consisting of one fighter squadron and one land-based attack squadron from the 1st Air Fleet to Rabaul.

Preparations for the Lae and Salamaua operations, which were unavoidably postponed after the loss of contact with the US task force, were recommenced. The most pressing preparation was the destruction of the anticipated build up of Allied air strength in eastern New Guinea. The 24th Air Flotilla conducted daily patrols in the waters south-east of the Solomon Islands and reconnaissance flights over eastern New Guinea and the Solomon Islands in search of the US carrier task force. In addition, operations to destroy Allied air strength in the eastern New Guinea area began on 24 February with the first combined fighter and bomber raid on Port Moresby.

The main feature of these raids was the destruction of land and sea facilities at the central base at Port Moresby. Aircraft stationed at the base were damaged and burned, weakening Allied strength in the region. Under these conditions, the transportation and landing of the invasion force to Lae and Salamaua was effected safely and successfully. Still, damage inflicted from air raids by the US task force was considerable.

Air battles over the bases from middle to late March</a>]

Elements of fighter units from the 4th Air Corps had advanced to Lae by the afternoon of 10 March following the speedy establishment of the base. Soon after, approximately half the strength of fighter and land-based attack units from the 4th Air Corps had been mobilised to Lae. The 24th Air Flotilla intensified its patrols in the Coral Sea area and its attacks against Port Moresby and the north-eastern sector of the Australian mainland.

On 14 March, nine land-based attack planes were directed to Port Moresby, while eight land-based attack planes and 12 Zero fighters attacked Australia for the first time, targeting Horn Island on York Peninsula at the north-eastern tip of Australia. No aircraft were engaged at Port Moresby. Thirteen Hawker Hurricanes and P-40s were engaged over Horn Island, with reports of nine aircraft shot down and three Lockheeds on the ground destroyed by fire. The airfield on Horn Island was crucifix-shaped and 1,200 metres long and 100 metres wide. It had not appeared in any intelligence until reference to it was found in documents taken from a Lockheed brought down at Gasmata on 3 March.

Reconnaissance was also carried out on Allied airbases at Townsville and Cooktown on the Australian mainland by two land-based attack planes on 21 March.

The 24th Air Flotilla participated in the offensive operations against the Solomon Islands in late March. In addition to conducting reconnaissance flights over key areas, flying boats and land-based attack planes attacked Tulagi on 17 and 18 March, though there was no significant resistance by the Allies to this raid.

The air war in eastern New Guinea and the Solomon Islands had proceeded well by March 1942, with Japanese forces establishing a central base at Rabaul and advance bases at Gasmata, Lae, and Salamaua. Port Moresby, the only remaining Allied base in the South Pacific Area, had been more or less contained.

The Allies, however, had initially intended to use Port Moresby simply as a relay station for their main bases on mainland Australia. Without effective attacks on these bases, the Japanese could not effectively assess damage to Allied air power and continual surveillance attacks on Port Moresby would become a necessity.

The Allies were simultaneously gradually expanding the base at Port Moresby and endeavouring to establish a base at Horn Island. The only airfields at Port Moresby by early February were at Kila and Bomana. By early March, however, more airfields were discovered on both banks of the Laloki River and on the right bank of the Goldie River, with airfields also being prepared approximately 50 kilometres to the north-west of Port Moresby.

No fighter aircraft were involved in the raids on Rabaul, Gasmata, Lae, and Salamaua up to this point, only large planes such as Lockheed Hudsons and B-17s. Neither were fighters in evidence in counter-attacks at Port Moresby. By mid-March, just as the Japanese had eased raids on Port Moresby, it seemed that Port Moresby had been reinforced with smaller aircraft. A land-based attack plane sent to Moresby for reconnaissance on 21 March failed to return.

At 6 am the following day, 22 March, nine Hawker Hurricane fighters and several Lockheed Hudson bombers raided Lae. Virtually the entire contingent of planes (nine Zeros and one land-based attack plane) were strafed on the ground and caught fire, and two Zeros were lost in the air. This was the first Allied raid against a Japanese base in which both fighters and bombers participated.

An assault force of 19 land-based attack planes and three Zero fighters from the 24th Air Flotilla set out for Port Moresby on 23 March, reporting a considerable force of small aircraft, and the bombing and burning of six small and one large aircraft. Thereafter, counter-attack by small aircraft during raids on Port Moresby became the norm, with increasing losses to the Japanese. Following from the build up of the US air force, the Allied powers had taken gradual steps towards a counter-offensive, using Port Moresby and Port Darwin as stepping stones in the South-East and South-West Areas, respectively.[41]

Advance of the 25th Air Flotilla to Rabaul

The Navy Department of Imperial Headquarters at that time assigned responsibility for air operations in the Inner South Pacific to the 24th Air Flotilla attached to the 4th Fleet as part of stage one of the campaign. It had raised the 11th Air Fleet (21st to 23rd Air Flotillas), an air formation for bases outside this line, to assist offensive operations in the southern area.

The 4th Fleet had advanced the main strength of the 24th Air Flotilla to Rabaul in response to the development of campaigns in the South-East Area. Ultimately, however, the flotilla had a responsibility, of increasing importance, for air operations in the Inner South Pacific, specifically in the Marshall Islands area. This point was especially driven home by the movements of the US task force in the waters of the Inner South Pacific Area.

The Navy Department of Imperial Headquarters redeployed its entire airbase units after the completion of stage one of the campaign. This important stage of the air campaign attempted to alter the situation in the entire Pacific area.

As a result, the 24th Air Flotilla was dedicated to duties in the Inner South Pacific Area, and the 25th Air Flotilla assigned to operations in the South-East Area.

The headquarters of the 25th Air Flotilla (commanded by Rear Admiral Yamada Sadayoshi) advanced to Rabaul on 29 March, and the formation was activated on 1 April 1942. The constituent units of the 25th Air Flotilla were the Yokohama Air Corps, which had previously participated in campaigns in the Rabaul area, the 4th Air Corps, and the Tainan Air Corps, the latter of which was diverted from campaigns in the Netherlands East Indies.[42]

UnitNewly formed strengthNumbers of crews
4th Air Corps36 land-based attack planes (Type-1, 2 in reserve)9 crews from 1st Air Corps, and 5 crews transferred from Kisarazu Air Corps on 1 April
Total of 37 crews
Tainan Air Corps45 fighters
6 land-based reconnaissance planes		31 crews transferred from 4th Air Corps on 1 April, reinforced with 3 crews from Japan
Yokohama Air Corps9 carrier-based fighters
12 flying boats		5 crews reinforcements for the present from Japan
Total of 13 crews including main unit and detachment

The commander of the Combined Fleet issued orders incorporating the 25th Air Flotilla into the formation of the South Seas Fleet (4th Fleet) effective from zero hour on 1 April. South Seas Fleet headquarters assigned the 25th Air Flotilla to duties that day as the "Bismarck Area Airbase Force". It inherited the duties of the 24th Air Flotilla Western Area Attack Force and was deployed in the Bismarck Archipelago area.

The current campaigns in the South-East Area were offensives against Port Moresby and Tulagi. Headquarters of the South Seas Fleet telegraphed orders to its constituent units on 4 April specifying early May for the Port Moresby invasion operation, thus granting authority for preparations.[43]

The air war of attrition intensifies</a>]

The aerial assault of Port Moresby by the 25th Air Flotilla began on 5 April. Its headquarters issued orders the previous day for the execution of repeated assaults on Port Moresby using most of its strength.

During the first two days, 5 and 6 April, combined attacks were carried out by land-based attack units from Rabaul and fighters from the Lae base. For the first of these attacks, nine Type-1 land-based bombers joined nine Zeros in the skies over Lae, and then carried out the raid on the Port Moresby airfields at 10.20 am. Seven attack planes (two turned back owing to engine trouble) rained shells over one large and one medium aircraft (possibly decoy planes), while the Zeros brought down two aircraft in aerial combat for the loss of one. A force of around ten Allied fighters was encountered.

On 6 April at 10.15, seven land-based attack planes (two turned back owing to engine trouble) and five Zeros carried out the raid and returned safely, reporting eight enemy aircraft down. There were 15 Allied fighters over the base.

Shortages of Zero crews on 7 April led to concerns over their ability to escort the attack planes. Consequently, a decision was made for a "fighter only" raid that day. However, the raid was cancelled when what were identified as eight American fighters attacked the Lae base at 6 am.

Bad weather delayed the resumption of raids on 9 April, but seven land-based attack planes and six Zeros attacked at 9.40 am on 10 April, reporting three Allied aircraft down for the loss of one Japanese plane.

In this manner, the assault on Port Moresby, the first real test of the 25th Air Flotilla, was not decisive, as it was conducted on a small scale and hampered by insufficient personnel. Further, the counter-attacks by Allied fighter planes in the skies over the Port Moresby base, and even in counter-offensives against Rabaul and Lae, became more formidable and gradually gained momentum into April.

The Allied attack on Lae by fighters at 3 pm on 4 April resulted in the destruction by fire of two Zeros, and damage by hits on eight Zeros and nine land-based attack planes. A further nine attack planes were hit at the time of the raid on 7 April by eight Allied dive-bombers.

At 1.20 on the afternoon of 9 April, four new-style bombers (judged to be B-26s with larger ejection tanks) raided the airstrip at Rabaul from an altitude of approximately 300 metres.

Rear Admiral Kanazawa, commander of the 8th Base Force, recorded the following in his diary for that day: "Suffered a severe raid from four English [sic] aircraft in the morning. At Vunakanau, 30 casualties from the 7th and 10th Establishment Squads, with one dead at the airfield under a torrent of exploding torpedoes. Conspicuous signs of defeat in the air war."
Sea-route invasion of Port Moresby

Difficult strategic conditions</a>]

As previously described, Imperial Headquarters issued orders at the end of January 1942 for a force comprising the South Seas Force and the 4th Fleet to "invade Port Moresby if at all possible". In fact, this offensive operation was not recognised as a serious difficulty given the conditions immediately after the invasion of Rabaul.

Meanwhile, as introduced above, the appearance of Allied air strength on mainland Australia and at Port Moresby, and the intensity in activity of the US task force, gave rise to conditions whereby Japanese army and navy commanders in the region were forced to adopt a measured attitude towards the invasion of Moresby.

Headquarters of the 25th Air Flotilla judged the total strength of Allied air power at the beginning of April in the South-East Area, including mainland Australia, to be approximately one thousand aircraft, including six hundred training planes. The US task force on manoeuvres in the Pacific was thought to be divided into two formations to the north and south. The southern group was formed around Saratoga using bases on mainland Australia and New Zealand.

Investigations were being carried out for the South Seas Force to land on the north coast of eastern New Guinea and invade Port Moresby overland. At the same time, Imperial Headquarters and navy units in the region were considering the original sea-route invasion plan.

The main difficulty for the sea-route invasion was that the transport convoy needed to pass along the south coast of New Guinea with one eye on the Allied airbases in northern Australia, some 480 kilometres distant from New Guinea’s eastern flank. However, within an arc some 440 kilometres from this area were interspersed a chain of numerous islands surrounded by coral reefs, severely limiting passage by ships.

The route to the Coral Sea needed to be either through the China Strait or the Jomard Passage, or by the longer detour to the east of the Louisiade Archipelago.

The shortest route was through the China Strait, which was mostly deep water. However, this option was deficient as the current was swift and there were many reefs. Further, it was extremely narrow, only allowing vessels to pass in single file.

The third option, the detour to the east of the Louisiades, was to be avoided as it would double the time to reach the target.

Ultimately, this left only the Jomard Passage as a route for the sea-bound invasion force. The distance from the passage to Port Moresby was around 650 kilometres.

This plan involved the invasion convoy departing Rabaul and steaming south for 780 kilometres before passing through the Jomard Passage. During the next 650 kilometres, the convoy was forced to conduct this operation across the sea, all the while susceptible to attack from Allied airbases at Townsville and Cooktown. If the Jomard Passage was navigated during the evening and the convoy maintained a constant 20 kilometres per hour, then it would be exposed in the Coral Sea during the following day for around 12 hours. This was the first promising opportunity to secure air and sea superiority in the Coral Sea.

The next problem was how to penetrate the barrier reef near the site of the planned offensive landing at Port Moresby. A world-famous barrier reef runs the length of the south coast of New Guinea. It continues from the eastern tip of New Guinea to near Port Moresby, running roughly parallel 3 to 16 kilometres from the coast in the shape of a natural fortress. Passage by landing craft is certainly restricted during low tide, but also during high tide.

The large Sinavi and Nateara reefs needed to be penetrated in order to land craft at Port Moresby. Three possible routes seemed available: the Liljeblad Passage, the Basilisk Passage, and Padana Nafua. The Liljeblad Passage was within firing range for the Paga Coastal Defence Battery established to the south of Moresby township. Further, the current was swift, and with sunken reefs in the passage and en route to the harbour, it was unsuitable for an operation with a large force.

Basilisk Passage was a standard waterway, but held the disadvantage that it opened directly on to the line of fire of the battery. Padana Nafua avoided these problems and was considered the most appropriate passage for the invasion of Port Moresby. Even so, a landing in the face of the enemy through a fortress-like barrier reef had not been planned since the opening of the war. This problem of penetration so close to enemy positions faced the Japanese forces.

If these strategic and technical problems could be overcome and Port Moresby could be invaded, then the great concern of maintenance of supply to hold a garrison force had to be addressed, along with unfavourable trends in Allied superiority of the air and sea. The South Seas Force, the unit given responsibility for garrison duties after the invasion, held grave concerns over the ability to maintain supply. The most pressing concern was the lack of water at Port Moresby, with supply needing to be transported many kilometres from inland areas. Consequently, the number of troops that could rely on rainwater was limited to approximately three to four thousand. The South Seas Force expressed the opinion that these limitations would severely hamper the garrison after the invasion.

Editor’s note: According to the memoirs of Lieutenant Colonel Akita Hiroshi, chief of intelligence in the Army Department of Imperial Headquarters, there also was a proposal for elements of the garrison force to prevent the enemy from using the area, while withdrawing the main strength of the force.

Preparations for the operation by the South Seas Force

The entire strength of the Horie Battalion of the South Seas Force assembled at Rabaul and proceeded with preparations for the invasion of Port Moresby after it had returned from the operations at Salamaua.

The air raids on Lae and Salamaua by the US task force on 10 March caused great anxiety in the commanders of the South Seas Force concerning the execution of the invasion of Port Moresby. Imperial Headquarters at that time had only given orders to "invade Port Moresby if at all possible." The overview and initiation of the operation was left to an agreement between local navy and army commanders. There was absolutely no discussion concerning concrete mechanisms to simply execute the operation.

At this time, the commander of the South Seas Force telegraphed the following appraisal to Imperial Headquarters on 20 March:[46]
These proposals were appropriate for the strategic conditions discussed above. Imperial Headquarters Army Department, possibly in response to the proposals, assigned the high-speed anti-aircraft ship Asakayama Maru to the South Seas Force. However, Asakayama Maru did not leave port at Moji for Rabaul until 25 April.

Editor’s note: A high-speed anti-aircraft ship was equipped with six anti-aircraft guns and assigned to protect a transport convoy. Eight of these were prepared after the outbreak of war.

At this time, South Seas Force headquarters had been researching strategic proposals for attacking Port Moresby. The following three proposals were most seriously considered: land-route over the mountains; barge mobilisation; and standard landing operation by convoy.

The land-route over the mountains proposal was identical to the plan that was later actually carried out. At this stage the proposal was only considered feasible if there was a road over the mountains; however reconnaissance to confirm that had not yet been undertaken. The commander of the South Seas Force sought opinions from his three battalion commanders, two of whom supported the land-route plan on the basis that it would be possible to advance at least half of the total troop strength to Port Moresby overland, rather than risking them all should the convoy be sunk in the Coral Sea.[47]

The barge mobilisation proposal involved landing troops in the eastern part of New Guinea, who would then carry out the invasion on Port Moresby after successive mobilisations along the south coast using landing barges. Landings would be conducted during the night to avoid attack from Allied aircraft. It was estimated that 5 nights would be required, based on nightly movements of approximately 110 kilometres. It was recognised that conducting these movements shoreward of the above-mentioned reefs would be difficult, so it was felt the advance would need to be carried out further to seaward.

It is clear that this mobilisation proposal would have been extremely difficult to execute. Nevertheless, the commander of the South Seas Force considered there was a chance of success, and judged the dangers of being sunk were higher during a standard transport and landing operation. Preparations for the mobilisation plan began to increase from early April. Research into methods of transporting food and munitions, embarkation rosters, cooking arrangements en route, and other aspects of the preparations proceeded.

In either case, the pressing task at hand was reconnaissance of the local area around Port Moresby. The commander of the South Seas Force sought the assistance of the navy prior to 15 April in the following tasks preparatory to the execution of the operation. These were relayed by the 24th to the 25th Air Flotilla:
As a result, army commanders accompanied several reconnaissance flights after 10 April. At that time, difficulties had arisen in the use of the Lae base by units of land-based attack planes, and counter-attacks by the Allies had continued to intensify.

Land-based attack plane units traversed the Owen Stanley Range at an altitude of 6,000 metres, flying reconnaissance and bombing missions over Port Moresby. Consequently, reconnaissance was largely ineffective, providing results that were less than anticipated.

Strengthening of the 4th Fleet

The 4th Fleet recognised the need for reinforcements to its air strength for the Port Moresby invasion operation, specifically for the despatch of an additional aircraft carrier, and entered into negotiations with the Combined Fleet. As a result, Shôhô, which was attached to the 4th Air Flotilla, was transferred to the 4th Fleet in the middle of March. Shôhô was a converted aircraft carrier with a complement of barely 28 planes, and was hardly suitable for the purposes of the army and navy units in the region. The Combined Fleet had completed invasion operations in the Netherlands East Indies on 10 March, and was currently planning offensive operations using a mobile carrier force in the Indian Ocean. As a result, it was judged that there was very little excess strength to be diverted to the South-East Area.

A Japanese carrier force sortied from Kendari on 28 March and carried out raids on Colombo on 6 April and on Trincomalee (east coast of Ceylon) on 9 April. The Combined Fleet progressed to stage two of the war with the completion of these attacks. Reorganisation of the formation was ordered on 10 April as the first phase of stage two operations. This governed the operations of the Combined Fleet until late May.[48]

The thrust of the reorganisation involved the main strength of the Combined Fleet assembling in the western part of the Inland Sea to prepare and train for the next phase of the campaign, which had two goals. The first involved the execution of the Port Moresby invasion in early May, and the associated strengthening of units in the South Pacific Area. The second, as previously discussed, involved diverting priority for airbase units to the Pacific region.

These orders of the Combined Fleet specified the requirements of the 4th Fleet for the Port Moresby operation in early May. They also strengthened the formation of the 4th Fleet with the addition of the 5th Squadron (less Nachi), Kaga (a large aircraft carrier), Zuiho (a seaplane tender), and the 7th and 20th Destroyer Squadrons. These units were to be attached to the 4th Fleet on 10 May, but this was brought forward to 20 April through specially issued orders.

The basic strength of the South Seas Fleet (4th Fleet) indicated in this order of battle was as follows:

Orders for this formation were issued on 5 April.[49]

However, even with these reinforcements, the 4th Fleet was still not satisfied. Further negotiations with the Combined Fleet resulted in "Operational telegraph order no. 109" being issued on 10 April for Kaga to be replaced by the 5th Air Flotilla (Shôkaku, Zuikaku).[50]

Strengthening of the 25th Air Flotilla

According to the new order of battle, the 11th Air Fleet (airbase units), based on the 21st (elements missing), 24th, 25th, and 26th Air Flotillas, was responsible for securing eastern New Guinea, the Bismarck Archipelago, the Marshall Islands, Wake Island, the East Caroline Basin, and the area around the Japanese homeland, as well as cooperating with the 4th Fleet (South Seas Fleet) and Fifth Fleet (Northern Fleet) in air campaigns in various regions.

The 24th and 25th Air Flotillas, which were attached to the South Seas Fleet, were returned to their original command.

The headquarters of the 11th Air Fleet moved to Tinian on 17 April.

Prior to this, on 10 April, the Airbase Force Command formed the 1st Air Attack Force from the 21st Air Flotilla, and the 4th, 5th, and 6th Air Attack Forces from the 24th, 25th, and 26th Air Flotillas, respectively. The 5th Air Attack Force was assigned to: "Patrol the region and destroy any invading enemy units; and destroy enemy air strength in the north-east area of Australia and cooperate with the Port Moresby invasion operation."

Meanwhile, the 25th Air Flotilla was strengthened by the Motoyama Air Corps (less its fighter elements). This unit had been assigned to the 22nd Air Flotilla in the South-West Area, but was especially transferred by the Combined Fleet for the Port Moresby invasion operation. Completion of the transfer was planned for 1 May.

Command of the 25th Air Flotilla issued on 11 April the following overview of the operational policy for the 5th Air Attack Force based on these new responsibilities:
An overview of the order of battle for the operational policy outline was issued on 10 April, as follows:

FormationCommandStrengthBaseMain responsibilities
1st ForceTainan Air Corps commander45 Zeros, 6 land-based reconnaissance planesLae, Rabaul (Lakunai)1. Aerial patrols over Lae and Rabaul areas
2. Destroy enemy planes in British New Guinea and north-east Australia
3. Cooperate with Moresby invasion
2nd Force4th Air Corps commander36 land-based attack planesLae, Rabaul (Vunakanau)1. Patrol the YZ patrol sector (see table below)
2. Destroy enemy planes in British New Guinea and north-east Australia
3. Stand by for attack by enemy fleet vessels
4. Cooperate with Moresby invasion
3rd ForceMotoyama Air Corps commander27 land-based attack planesRabaulAs above
4th ForceYokohama Air Corps commander12 large flying boats
9 float planes		Rabaul1. Patrol the patrol sector
2. Cooperate with Moresby invasion
3. Elements to stand by for contact
Special Duty ForceMogamikawa Maru commanderMogamikawa MaruTransport according to special orders
This would seem to indicate that the strategic position of airbase units had steadily been strengthened in the South Pacific Area. However, what was the actual situation? After the reorganisation, the front-line strength of the relatively new 25th Air Flotilla was quite different from its listed complement.

While the main strength of the Yokohama Air Corps had advanced to Rabaul by 5 April, it would take time to assemble at Rabaul the elements remaining in the Marshall Islands. Elements of the Tainan Air Corps, consisting mainly of fighters, which had been temporarily transferred to the 4th Air Corps, finally arrived in Rabaul on 16 April owing to shortages of transport ships.

Although preparations were quickly undertaken after reinforcements arrived – 12 Zeros on Goshû Maru and 24 Zeros on Kasuga Maru arrived on 7 and 12 April, respectively – numbers were insufficient and the 4th Air Corps never reached a full complement. Nine Zeros which had been transferred to the 1st Air Corps from among the strength of the 4th Air Corps, which consisted mainly of land-based attack planes, were returned to Rabaul on 2 May. This was in addition to some Type-1 land-based attack planes that were transported to Japan for training. Nine land-based attack planes of the Motoyama Air Corps (fighter elements missing) advanced to Rabaul on 20 April. The main strength (18 attack planes), however, did not arrive until 1 May.

The working complement of aircraft from 1 April to 1 May 1942 was as follows. Crew numbers generally were greater than the number of aircraft:[51]

DateFighters (Type-96 Zero)Land-based attack planes (Type-1 and Type-96)Flying boats
1 April151611
5 April131313
10 April12813
15 April8912
20 April312514
25 April292014
1 May234213

Editor’s note: The patrol sectors at that time were as follows.[52]

Patrol sectorBaseAreaRangeAircraft
PRabaul45–75 degrees1,100 kilometresTwo per sector as a general rule
QRabaul75–105 degrees"
RRabaul135–165 degrees"
YLae130–160 degrees"
ZLae160–190 degrees"
Air battles in April 1942

Allied air attacks on Rabaul and Lae intensified after the beginning of April. The powerful raid by the impressive B-26s on 9 April in particular has already been mentioned. Formations of six or seven B-26s also conducted raids on 11, 12, 18, and 19 April. This was the start of raids on Rabaul by US Army Air Force medium bombers. The lead plane during the raid on 18 April was shot down. Intelligence gained from the two crew members indicated the raid had originated from a US base in eastern Australia. As this intelligence was on the whole deemed to be accurate, the following report was telegraphed immediately to Imperial Headquarters:[53]

Meanwhile, Lae suffered additional attacks from combined fighter and bomber groups. Eight bombers and eight fighters raided on 11 April, and six fighters and four bombers raided again on 13 April. Japanese fighter units scrambled and reported shooting down around one-third of the raiders on each occasion. Damage to Japanese aircraft was not slight, however, and the land-based attack planes in the area were ordered to evacuate to Rabaul on 11 April. Thereafter, patrols over the Y sector, which was the responsibility of land-based attack planes at Lae, were conducted from the base at Rabaul.

In this way, Japanese air attacks on Port Moresby after 10 April were postponed for around a week in order to repair aircraft. Commander Yamada of the 25th Air Flotilla lamented that "The number of serviceable aircraft for attacks on Port Moresby on 14 April did not exceed three fighters and three attack planes." At the same time, General MacArthur in Australia declared that "The Japanese have suffered a deadly blow to their aircraft numbers, and are currently unable to mount an attack."

During this time, the 25th Air Flotilla undertook the following reconnaissance in preparation for attacks on Port Moresby:

The 25th Air Flotilla had reached its lowest strength by 13–14 April. However, it unexpectedly reached a state of preparedness after the completion of the assembly at Rabaul of the Tainan Air Corps on 16 April, and through efforts to quickly prepare Zeros that had arrived as reinforcements. The 25th Air Flotilla began full-scale raids on Port Moresby on 17 April after a planned raid the previous day was cancelled owing to inclement weather. Raids were conducted by a force of 15 fighters and seven land-based attack planes (two returned with malfunctions en route), with reports of five Allied planes downed for the loss of one fighter.

Raids were carried out almost daily thereafter until the beginning of May. Of particular note were raids in two or three waves. Horn Island was attacked on 30 April using six fighters and eight attack planes. Tulagi was also attacked on 25 and 30 April with land-based attack planes and large flying boats.

Allied counter-attacks also were persistent. Daily raids were directed exclusively at Lae after the attacks on Rabaul on 22 and 23 April. However, these raids were only conducted with bombers, such as B-17s, B-26s, and Lockheeds, in formations of two or three planes. This led the commander of the 25th Air Flotilla to judge that Allied air strength at Port Moresby had been smashed by 29 April.[54]

Nevertheless, the very next day, on 30 April, Lae was raided by a formation of three B-26s bombers and twelve P-39 fighters. It was clear that the Allies had reinforced Moresby with a considerable number of P-39s. Further, the discovery of the impressive B-25s at Horn Island led to the inevitable conclusion that the build up of the US Army Air Force in the Australian region was continuing and intensifying.

The sea-route offensive against Port Moresby was to have been carried out under these conditions, but the course of events during the air war to smash the enemy in eastern New Guinea and the region in the north-east of Australia had introduced an element of doubt.

Navy plans for the offensive</a>]

Confidence in the execution of the operation firmed within headquarters of the 4th Fleet after the 5th Squadron (Nachi missing) and the 5th Air Flotilla were attached to the formation. Offensive plans were formulated and preparations for the operation continued.

As indicated by the above quoted army and navy central agreement, the 4th Fleet still had on its plate the invasion of the strategically important Tulagi in the southern Solomon Islands in addition to the Port Moresby operation.

The occupation of Shortland Island, a relay base for the Tulagi offensive, was completed by late March.

In addition, the 4th Fleet was responsible for offensives against Nauru and Ocean Island in the Gilbert Islands. These operations had been previously planned primarily to secure supplies of phosphorus.

These offensives in three different areas were planned together by the 4th Fleet, which named the entire operation the Moresby Operation (MO Operation). Samarai, a small island of some 4 kilometres circumference overlooking the China Strait, was the administrative centre for the area and a key transport link. As previously mentioned, the passages through the Louisiade Archipelago were extremely few. The occupation of Samarai to secure the China Strait was consequently of great importance. The 4th Fleet planned for elements of the special naval landing force from the Port Moresby invasion to occupy the island.

The objectives of the Moresby Operation, as formulated by the 4th Fleet, were as follows:
The 4th Fleet planned successive offensives against Tulagi on 3 May, Port Moresby on 10 May, and Nauru and Ocean Island on 15 May. An order of battle was prepared for these operations, as well as for the occupation of Samarai on 12 May. Further, the commander of the 4th Fleet divided the order of battle into the period up to the completion of the Tulagi offensive (A) and the period thereafter (B). "Secret South Seas Fleet operational order no. 13" was subsequently issued on 23 April. An overview of this order is as follows:

FormationCommand
(4th Fleet)		StrengthMain responsibilities
Moresby (MO) Main Force6th Squadron commander6th Squadron (Aoba, Kako, Kinugasa, Furutaka), Shôhô, Sazanami (7th Destroyer Squadron)
Tulagi Invasion Force (RXB)19th Squadron commander19th Squadron (Okinoshima (Tsugaru and Tokiwa missing))**
Half of 23rd Destroyer Squadron (Kikuzuki, Yûzuki)**
14th Minesweeper Flotilla (Tama Maru, Hagoromo Maru, No. 2 Noshiro Maru)*
No. 1 and No. 2 Special Minesweeper
56th Submarine Chaser Squadron (No. 8 Tama Maru, No. 3 Toshi Maru)
Kure 3rd Special Naval Landing Party (part strength)***
Azumayama Maru*, Kôei Maru
7th Establishment Squad (part strength)***		1. Tulagi offensive at dawn 3 May
2. Establishment of airbase
Moresby Invasion Force (RZP)6th Torpedo Squadron commander6th Torpedo Squadron (Yûbari, 29th Destroyer Squadron (Oite, Asanagi), 30th Destroyer Squadron (Muzuki, Mochizuki, Yayoi), 23rd Destroyer Squadron (Uzuki (Yûnagi, Kikuzuki and Yûzuki missing))
Tsugaru
No. 20 Minesweeper
Kure 3rd Special Naval Landing Party (elements missing)
Azumayama Maru, Goyô Maru, Akihasan Maru, Shôkai Maru, Chôwa Maru, Oshima, No. 2 Fumi Maru, No. 3 Seki Maru, Mogamikawa Maru
10th Establishment Squad		1. Direct protection of South Seas Force
2. Moresby invasion at dawn 10 May
3. Establishment of airbase
4. Secure supply route
5. Protect transport route
Covering Force18th Squadron commander18th Squadron (Tenryû, Tatsuta)
Kiyokawa Maru air corps
Kamikawa Maru
Two specially fitted gunboats from 5th Gunboat Squadron (Nikkai Maru, Keijô Maru)
Shôei Maru
14th Minesweeper Flotilla part (Hagoromo Maru, No. 2 Noshiro Maru)
Kure 3rd Special Naval Landing Party (part strength)		1. Protection of the Tulagi Invasion Force
2. Protection of the Moresby Invasion Force and escort for the South Seas Force
3. Establishment of invasion bases at key areas
4. Samarai offensive on 12 May
Raiding Force21st Submarine Group (Ro-33 and Ro-34)1. Seek and attack enemy fleet
2. Surveillance lead vessel in Moresby harbour
Supply UnitsIshirô, Hôyô MaruSupply
Moresby (MO) Carrier Force5th Squadron commander5th Squadron (Myôkô, Haguro (Nachi missing))
5th Air Flotilla (Zuikaku, Shôkaku)
27th Destroyer Squadron (Shigure, Yûgure, Ariake, Shiratsuyu)
7th Destroyer Squadron (Shioakebono (Sazanami missing))		1. Destroy enemy vessels
2. Destroy enemy air strength
Nauru and Ocean Island Invasion Force19th Squadron commander19th Squadron (Okinoshima (Tsugaru and Tokiwa missing))***
23rd Destroyer Squadron (Kikuzuki, Yûzuki)***
Kinryû Maru, Takahata Maru
6th Base Naval Landing Party, Kashima Naval Landing Party (separate orders on formation)
(after 11 May) Tatsuta, Tsugaru		Nauru and Ocean Island offensives on 15 May
(Editor’s note: Okinoshima was sunk on 11 May so replaced with Tatsuta)
Bismarck Area (R) Defence Force8th Base Force commander8th Signals Unit
8th Submarine Base Force
81st Garrison Rabaul Unit
5th Gunboat Squadron (Seikai Maru)
56th Submarine Chaser Squadron (No. 5 Kotobuki Maru)
Notes: The Tulagi Invasion Force was dispersed according to orders issued for the "B order of battle", indicated as follows: "*" to Moresby Invasion Force; "**" to Nauru and Ocean Island Invasion Force; and "***" to Bismarck Area Defence Force.

The central command unit as indicated in the order of battle, taking into account the general situation, was naturally the Moresby Carrier Force. The commander of the 4th Fleet issued instructions to the carrier force through the above-mentioned orders issued on 23 April, as follows:
The Tulagi and Moresby Invasion Forces assembled and made preparations at Rabaul, while other units did the same at Truk. Departure of the units from Rabaul and Truk was planned as follows, according to the B order of battle issued on 4 May:

South Seas Force plans for the offensive</a>]

The South Seas Force had been leaning towards adopting a barge mobilisation plan for the invasion of Port Moresby. The strengthening of the 4th Fleet, however, had increased confidence in the success of a standard landing-style operation.

Strategic planning and discussions for the Moresby invasion were held at 4th Fleet headquarters in Truk on 16–17 April, with the participation of Lieutenant Colonel Tanaka, the chief of staff of the South Seas Force. Meanwhile, with a superior navy escort strength in the region, and with three carriers en route to the Coral Sea to engage in direct support, optimism was mounting that the campaign would be successful.

Until this time, the Army Department of Imperial Headquarters had not given any specific instructions or leadership concerning the Port Moresby campaign. Consequently, chief of staff Tanaka wrote to Imperial Headquarters from Truk outlining the commander’s appraisal of the campaign and seeking instructions from central headquarters.

The reply, addressed to the South Seas Force commander, was telegraphed on 18 April. The main thrust was as follows:
Meanwhile, Imperial Headquarters had advised of the despatch of the high-speed anti-aircraft ship Asakayama Maru. In addition, the opinion was submitted that "the navy has judged that it will be possible to compensate for the lack of speed in the convoy by the use of the large force planned for the current campaign".

At that time, the Army Department of Imperial Headquarters was not particularly enthusiastic about the Port Moresby invasion operation. As will be discussed below, they became confrontational over the operation to blockade the supply route between the United States and Australia, and over the operations to invade Midway and the Aleutian Islands.

An agreement was reached between the army and navy at the 8th Base Force headquarters in Rabaul on 25 and 26 April. Details were fleshed out by 3 May.

The transport convoy consisted of a total of 11 ships. The army had five in addition to Asakayama Maru, and the navy six, including Azumayama Maru. The route adopted for the convoy was as follows: depart Rabaul, then after passing through the St George’s Channel, proceed south-south-west in the seas to the west of Bougainville, turn south-west when to the east of Woodlark Island and head towards Deboyne Island, and then enter the Coral Sea through the Jomard Passage. The support group had occupied Deboyne Island on 4 May with plans to construct a seaplane base for the Moresby operation.

The commander of the South Seas Force issued the order of battle for the Port Moresby invasion on 29 April. This outlined a movement of barges outside of the reef, with the main force landing at Taurama after passing through Padana Nafua passage, and elements (based on 1st Battalion of 144th Infantry Regiment) passing through the Liljeblad Passage and landing on the coast at Oiso. These landings were to be completed by early dawn on 10 May, followed by offensives against Kila airfield and the Moresby peninsula. Finally, attacks against the airfields to the north were planned.

The airfields, particularly those to the north of Port Moresby, were the main objective of the offensive.

Intelligence from the start of the war confirmed the existence of a garrison of approximately battalion strength to the north of Port Moresby township. Accurate information concerning establishments and army strength was not available, but it was estimated that the Allied land strength was approximately five thousand men.[57]

The 25th Air Flotilla’s operational plan</a>]

The 25th Air Flotilla had intensified its air raids against Port Moresby and in the northern Australian area since early April in preparation for the Port Moresby invasion operation.

As the operation finally approached, the 25th Air Flotilla was given the following responsibilities:
FormationCommand and strengthBase to utiliseMain responsibilities
1st ForceTainan Air CorpsLakunai, Lae, Moresby, Kila1. Smash air strength in New Guinea and north-east Australia
2. Patrol bases and skies over convoy
3. Z duties
2nd Force4th Air CorpsVunakanau, Lae, Moresby1. (As above)
2. Patrolling
3. Attack enemy task force
4. Y duties
5. Assist land operations
3rd ForceMotoyama Air CorpsVunakanau1. Patrolling
2. Attack enemy task force
3. Smash air strength in New Guinea and north-east Australia
4th ForceYokohama Air CorpsTulagi, Shortland Island, Deboyne Island1. Patrolling
2. Prepare for contact
3. Attack enemy task force
Special Duty ForceMogamikawa Maru1. Transport
2. Provisions
PatrolBaseExtentNotes
ALae124–190 degreesSearch lines no. 1–5
BRabaul135–172 degreesSearch lines no. 1–4
CRabaul53–115 degreesSearch line no. 1–4
DShortland Island100–194 degreesSearch line no. 1–4
EShortland Island45–100 degrees
FTulagi95–225 degreesSearch line no. 1–11
GTulagi45–95 degreesSearch line no. 1–5
IDeboyne Island155–180 degreesSearch line no. 1, 2
The range of all patrols was 1,100 kilometres.

Departure of the fleet

The Tulagi Invasion Force (based on the Kure 3rd Special Naval Landing Party) occupied Tulagi as planned without resistance at dawn on 3 May.

The South Seas Force embarked at Rabaul harbour during 2 and 3 May and set out south at 4 pm on 4 May (Asakayama Maru left on 5 May) under the protection of the main strength of the 4th Fleet in high spirits for adventure.[59]

This formation unexpectedly encountered the US task force two days after setting sail and engaged in the battle of the Coral Sea, the first ever carrier-based naval air battle. The formation returned to Rabaul with their offensive capability intact.

The sea-route operation against Port Moresby required information about enemy strengths and deployments. This information was contained in unit orders for the Port Moresby operation (issued on 23 April), as follows:

Outline of the battle of the Coral Sea

The situation before the battle

Airbase units intensified their daily attacks during April and into early May.

The field strength of the 5th Air Attack Force (25th Air Flotilla) on 1 May was 28 Zero fighters (18 in service), 11 Type-96 fighters (6 in service), 28 Type-1 land-based attack planes (17 in service), 26 Type-96 land-based attack planes (25 in service), and 16 Type-97 large flying boats (12 in service).[60]

It had been judged that the use of small-scale aircraft based at Port Moresby had been suppressed by 4 May. However, there was no means to continue attacks on the logistics bases after this time, so the control of the air could not be maintained. Further, the range of daily patrols seeking enemy incursions had widened, finding no trace of the enemy in the seas in the region from the Coral Sea to the Solomon Islands.

Meanwhile, the Tulagi operation had proceeded favourably on 3 May as planned. Consequently, some of the flying boats were advanced to Tulagi that same afternoon. However, the force was attacked suddenly by a US task force of approximately eighty carrier-based aircraft the following day for about 6 hours from 6.30 am. The raid resulted in the sinking of one destroyer, two minesweepers, and one submarine chaser.

At that time, the Port Moresby operation carrier force had commenced refuelling approximately 670 kilometres north of Tulagi. It had been delayed in sailing south because the Zeros, which were aboard the fleet as replacements, were unable to be transported to Rabaul owing to poor weather.

Raids on Tulagi by carrier-based aircraft meant that the US task force had advanced into the region. The opinion from within Combined Fleet headquarters was as follows:

Having confidence in the strength of the 5th Air Flotilla, it will be possible to catch and destroy the enemy aircraft carriers that have eluded us since the beginning of the war. In addition to bringing us great joy, this will inform the enemy appropriately of our strength. The South Seas Fleet has yearned for suitable hard fighting, given that there was a good chance of receiving raids from enemy carriers up until the completion of the Port Moresby operation.[61]

However, rather than advancing air units, the 5th Air Flotilla feared the Allies and withdrew, concerned that they would be unable to defeat the US carrier fleet:[62]
2. The Port Moresby Carrier Force will immediately proceed towards and destroy the enemy task force.[63]

Upon receipt of this telegraph, the carrier force (5th Air Flotilla) immediately ceased refuelling and steamed south. Submarine units immediately proceeded to the deployment line.

The South Seas Fleet (4th Fleet) remained confident in its air strength, so decided to proceed as planned with the invasion of Port Moresby. The Tulagi Invasion Force was withdrawn to the north during the raids, but the Port Moresby Invasion Force was ordered to depart from Rabaul according to the plan.

The 5th Air Attack Force and the 5th Air Flotilla hunted for the Allied task force on 5 May without any success. The Japanese carrier force headed north-east from its position to the south of Guadalcanal, while the invasion force proceeded as planned to the south. Meanwhile, the US task force, which was based on two aircraft carriers, assembled approximately 670 kilometres south of Guadalcanal. Owing to refuelling, only the carrier Yorktown participated in the Tulagi attack, after which it headed south to rendezvous with the main force. The movements of the Japanese Port Moresby Invasion Force were carried out according to the offensive plan.[64]

A report was received at 8.10 am on 6 May from a reconnaissance plane from the Yokohama Air Corps: "Sighting of what seems a large enemy force. Bearing 192 degrees from base, distance 780 kilometres."[65]

The Port Moresby Invasion Force was even at that time refuelling west of Guadalcanal, but turned south at 10 am immediately upon receiving this report.

However, the reconnaissance plane lost contact with the US task force at midday. The distance between the two forces was felt to be around 670 kilometres. Vice Admiral Takagi, the commander of the carrier force, determined that the opportunity of an attack during the day had been lost, so turned north at 6 pm to wait for a chance to attack during the following day. Takagi considered that: "The enemy may attack our invasion force on 7 May, but the chances are good that it will be steaming south on 7 May for an attack on 8 May. Consequently, it is essential that we seek out and attack the task force during 7 May." Patrols were sent to the estimated position of the US task force during the morning of 7 May. The commander had decided that, if it was not possible to locate the enemy, the group would proceed west into the Coral Sea.

Meanwhile, the Japanese invasion force had continued from the previous day to steam south in the central Coral Sea, but made contact with Allied planes just after 9 am. An attack from the US task force now seemed inevitable, so patrols were placed on high alert.

Airbase units deployed at midday after a large flying boat had made contact with the task force. However, preparations for the planned attack by flying boats were not ready and the attack was not possible. Units of the escort fleet had arrived at Deboyne Island as planned and begun preparations for the seaplane base.

The US task force had judged that the Japanese invasion force would pass through the Jomard Passage on 7 or 8 May, but had not yet determined the exact position of the Port Moresby Carrier Force.

The battle on 7 May

Finally, as 7 May arrived, the convoy planned to exit the Jomard Passage into the Coral Sea, while the Moresby Main Force and the Port Moresby Escort Force had left Deboyne Island and were deployed in the waters to the north of Rossel Island. The Port Moresby Carrier Force was some 560 kilometres to the south-west looking for the US task force that had appeared during the previous few days.

On the other side, Allied units were carrying out raids using airbase units, while the task force had left its course to the north-west to pursue the Japanese invasion force in the waters to the south of Rossel Island.

Contact and engagements by each side to the east and west occurred almost simultaneously.

First, search planes from the 5th Air Flotilla discovered at 5.22 am an enemy formation based on one aircraft carrier situated 740 kilometres south-west of Tulagi. Rear Admiral Hara, commander of the 5th Air Flotilla, immediately ordered a full strength attack. The attack force (approximately 78 planes) left the carriers between 6 and 6.15 am.

Next, a report was received from a reconnaissance seaplane at around 6.40 am that "one battleship, one cruiser, seven destroyers and what looks like one aircraft carrier discovered at bearing 170 degrees, range 152 kilometres from Rossel Island".[66]

Command of the 4th Fleet, on hearing these reports, judged that the US task force had broken into two formations. A decision was made first to attack the formation near Rossel island. The full strength of their force was mobilised at 8.45 am.[67]

In accordance with a judgment of the conditions, the invasion force, under the protection of the escort fleet, began to withdraw to the north-west.

The attack units of the 5th Air Flotilla arrived at the target area under the guidance of the contact plane to find that the aircraft carrier had gone, leaving an oil tanker and one destroyer. The attack unit split into two formations and searched the area for over 2 hours, but was unable to locate the aircraft carrier. The carrier-based attack planes bombarded the tanker and nearby destroyer and then returned.

Meanwhile, attacks had begun on the transport convoy to the west from aircraft of the US task force. Attack units from Lexington departed at 7.26 am, and from Yorktown at 8 am (total of 93 planes).

US torpedo bombers concentrated on the Japanese carrier Shôhô, which was providing direct support to the convoy. It sunk at 9.35 am at a position 59 degrees and 96 kilometres from Deboyne Island. The receipt of the report, "Shôhô has sunk", was a great shock to the commanders of the South Seas Fleet.[68]

As previously mentioned, the Port Moresby Carrier Force had been delayed from loading the first attack group, but was now steaming south at full speed. Unfortunately, the 5th Air Flotilla had been formed just prior to the start of the war. Its crew were insufficiently trained and had great difficulties with night operations. Consequently, only experienced crews left the carrier after 2 pm for twilight battles. These units were unable to locate the task force so dumped their bombs and torpedos and headed back to the carriers. An enemy carrier was located, however, near to the Japanese carrier, but by that time the attack units could provide no assistance.

The commander of the Combined Fleet stated the following in his official diary for this day:
The South Seas Fleet received the report that contact had been lost with the Allied task force, which was at that time some 560 kilometres distant. Recognising that a night battle would be difficult, the decision was taken at 6.20 pm to cease the attack and postpone the invasion of Port Moresby for two days.[69]

The battle on 8 May

Search planes, which lifted from Shôkaku at 4 am on 8 May, located the US task force at a bearing of 205 degrees and a range of 435 kilometres. Its heading was 170 degrees at a speed of 30 kilometres per hour.

An attack unit from the 5th Air Flotilla sortied at 7.15 am. Its strength totalled 69 aircraft, including 18 fighters, 33 bombers, and 18 attack planes. The Japanese carrier fleet was speeding south at 55 kilometres per hour ahead of a squall.

Meanwhile, contact was made by US search planes at about 6.30 am, and thinking this was an early morning raid by an enemy formation, the Japanese went to high alert. Concentrated attacks by US aircraft began at 8.50 am. By 9.40 am, Shôkaku had received four waves of attacks, which left it on fire and unserviceable by aircraft.

Meanwhile, the Japanese attack group had reached the target and successfully undertaken an air strike at 9.20 am. At 9.25 am, a report was sent that read: "Saratoga attacked and sunk." The attack units were taken on board the remaining carrier Zuikaku. It was confirmed that as a result of this battle, Saratoga and Yorktown had been sunk, and heavy damage inflicted to one battleship and one heavy cruiser. The entire complement of the attack force had received damage from shellfire.

The commander of the Port Moresby Carrier Force was determined to conserve fuel and offensive strength and was apprehensive about high-speed travel in night battles. Consequently, he telegraphed the following message to the commanders of the South Seas Fleet at 12.30 pm: "The assault units returned at around 1100 hrs. Owing to the need for repairs, a second attack today is unlikely."

After taking on board the assault units, the Port Moresby Carrier Force turned and headed north at about 1 pm. At that time, only two vessels from the 5th Squadron could undertake night operations. The commander of the carrier force, Vice Admiral Takagi, received a telegraphic order from the South Seas Fleet at 2 pm to "Cease offensive and head north." The group reluctantly disengaged from the enemy to refuel and undertake repairs to aircraft.[70]

The commander of the 4th Fleet (South Seas Fleet) telegraphed the commander of the South Seas Force, as follows: "The day for the offensive against Port Moresby has been revised to day x+2."

Tsugaru, the relay ship for this message, received the telegraph at 7.30 pm on 7 May, but did not relay it to the commander of the South Seas Force on board Matsue Maru until 5.30 am the following day.[71] This delay is thought to be because of a wish to avoid signal flares at night.

The South Seas Force commander relayed the following back to Tsugaru: "First, I have been informed of the delay of the invasion day. Secondly, I would like information on yesterday’s battles." The reply from Tsugaru was telegraphed:
End of the naval battle

The South Seas Fleet for a time disengaged from the enemy and headed north. At 9 am on 8 May, orders were received from the Combined Fleet: "At this time, destroy the powerful remaining enemy force."[72] The fleet immediately turned south and commenced searching, but at this time it was not possible to locate the Allied task force.

The commander of the South Seas Force received the following message just after 3 pm on 8 May from the commander of the 4th Fleet via the captain of Tsugaru: "The Port Moresby operation is postponed and the convoy will return to Rabaul. Be advised that your unit will land and return to their barracks." The commander replied. "First, I offer my deepest congratulations for such an unprecedented victory. Secondly, I confirm that I understand we will return to Rabaul."[73] Under these circumstances, the Port Moresby invasion convoy returned to Rabaul, all the while under high alert. The 30th Destroyer Squadron joined the convoy at 9.40 am and was deployed defensively prior to entering the St George’s Channel owing to fears of the presence of US submarines.

The convoy safely steamed through the channel and entered Rabaul at 3 pm.

The Combined Fleet command, which was considering the date of the next invasion attempt, issued orders at 1.30 pm on 10 May advising that, "The Port Moresby operation will be postponed until phase three (July)." Further, the chief of staff advised that, "If there are no signs of the enemy today, the Port Moresby Carrier Force (5th Squadron and 5th Air Flotilla) will be removed from the South Seas Fleet." This was effectively the end of the Port Moresby sea-route invasion.

An overview of the losses on each side during this naval battle was as follows:

TypeJapaneseUnited States
Ships1 carrier (Shôhô)
1 destroyer
4 specialty vessels
Total 6 sunk		1 carrier (Lexington)
1 oil tanker
1 destroyer
Total 3 sunk
Aircraft8066
PersonnelApproximately 900543

Imperial Headquarters instructed the commander of the South Seas Force that: "The execution of the Port Moresby invasion operation has been temporarily postponed. The force will be transferred to the command of the newly established 17th Army. The operation will be executed around July."[74]

Notes

1 Sanagi Kowashi, Sanagi Kowashi Chûsa nisshi (Diary of Lieutenant Colonel Sanagi Kowashi).
2 Kanezawa Masao, Kanezawa nisshi (Diary of Rear Admiral Kanezawa Masao).
3 Imoto Kumao, Imoto Kumao Chûsa nisshi (Diary of Lieutenant Colonel Imoto Kumao).
4 Dairikumei, Daikairei, Dairikushi, oyobi Daikaishi no kakutsuzuri (Great Army Orders, Great Navy Orders, Great Army Instructions, and Great Navy Instructions).
5 Sakusen kenkyû shiryô (Operations research documents).
6 Sakusen kenkyû shiryô (Operations research documents).
7 Okamoto Harutoshi, Okamoto Harutoshi Shôsa no kaisô (Recollections of Lieutenant Commander Okamoto Harutoshi).
8 Okamoto Harutoshi, Okamoto Harutoshi Shôsa no kaisô (Recollections of Lieutenant Commander Okamoto Harutoshi).
9 Kôdô chôsho (Surveys of movements).
10 Kôdô chôsho (Surveys of movements).
11 Okamoto Harutoshi, Okamoto Harutoshi Shôsa no kaisô (Recollections of Lieutenant Commander Okamoto Harutoshi).
12 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
13 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
14 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru); and Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
15 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
16 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
17 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
18 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
19 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
20 Kôdô chôsho (Surveys of movements).
21 Nankai Shitaisen shi shiryô (Historical documents of the South Seas Force).
22 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
23 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
24 Toyofuku Tetsuo, Toyofuku Shôsa kôwa (Interview with Major Toyofuku Tetsuo).
25 Nantô Hômen jûyô denpô tsuzuri (Important telegrams of the Southern Area Army).
26 Kôdô chôsho (Surveys of movements).
27 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
28 Okamoto Harutoshi, Okamoto Harutoshi Shôsa no kaisô (Recollections of Lieutenant Commander Okamoto Harutoshi).
29 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
30 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
31 Kanezawa Masao, Kanezawa nisshi (Diary of Rear Admiral Kanezawa Masao).
32 Kôdô chôsho (Surveys of movements).
33 Louis Morton, Strategy and command: the first two years (Washington: Department of the Army, 1962), p. 205; and Lionel Wigmore, The Japanese thrust (Canberra: Australian War Memorial, 1957).
34 Frederic C. Sherman, Combat command: the American aircraft carriers in the Pacific War (New York: Dutton, 1950).
35 Kanezawa Masao, Kanezawa nisshi (Diary of Rear Admiral Kanezawa Masao).
36 Louis Morton, Strategy and command: the first two years (Washington: Department of the Army, 1962), p. 202.
37 Kanezawa Masao, Kanezawa nisshi (Diary of Rear Admiral Kanezawa Masao).
38 Kôdô chôsho (Surveys of movements).
39 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru); and Frederic C. Sherman, Combat command: the American aircraft carriers in the Pacific War (New York: Dutton, 1950).
40 Kôdô chôsho (Surveys of movements); and Kanezawa Masao, Kanezawa nisshi (Diary of Rear Admiral Kanezawa Masao).
41 Dudley McCarthy, South-West Pacific – first year: Kokoda to Wau (Canberra: Australian War Memorial, 1959), p. 67.
42 Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla); and Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
43 Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
44 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi); and Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
45 Nankai Shitai senshi shiryô (Historical documents of the South Seas Force); and Toyofuku Tetsuo, Toyofuku Shôsa kôwa (Interview with Major Toyofuku Tetsuo).
46 Nantô Taiheiyô Hômen kankei denpô tsuzuri (Telegrams related to the South-East Pacific Area).
47 Nankai Shitai senshi shiryô (Historical documents of the South Seas Force).
48 Dairikumei, Daikairei, Dairikushi, oyobi Daikaishi no kakutsuzuri (Great Army Orders, Great Navy Orders, Great Army Instructions, and Great Navy Instructions).
49 Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
50 Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
51 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
52 Dai 24 Kôkû Sentai sentô shôhô (Detailed battle reports of the 24th Air Flotilla).
53 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
54 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
55 Sakusen kenkyû shiryô (Operations research documents).
56 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
57 Nankai Shitai senshi shiryô (Historical documents of the South Seas Force).
58 Nankai Shitai sakusen shiryô (South Seas Force operations documents); and Dai 25 Kôkû Sentai senji nisshi (War diary of the 25th Air Flotilla).
59 Daitôasen shi Minami Taiheiyô sakusen (Greater East Asian War South Pacific operations).
60 Yamada Sadayoshi, Yamada nisshi (Diary of Rear Admiral Yamada Sadayoshi).
61 Ugaki Matome, Sensô roku (A record of war).
62 Hara Chûichi, Hara Chûichi Shôshô no kaisô (Recollections of Rear Admiral Hara Chûichi).
63 Tomioka Sadatoshi, Tomioka Sadatoshi Taisa no kaisô (Recollections of Colonel Tomioka Sadatoshi).
64 Dudley McCarthy, South-West Pacific – first year: Kokoda to Wau (Canberra: Australian War Memorial, 1959), p. 80.
65 Gunkan Zuikaku sento shôhô (Detailed battle reports of the warship Zuikaku).
66 Gunkan Zuikaku sento shôhô (Detailed battle reports of the warship Zuikaku).
67 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
68 Yamaguchi Moriyoshi, Yamaguchi Moriyoshi Chûsa no kaisô (Recollections of Commander Yamaguchi Moriyoshi).
69 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
70 Dai 5 Sentai sentô shôhô (Detailed battle reports of the 5th Squadron).
71 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
72 Dai 5 Kôkû Sentai senji nisshi (War diary of the 5th Air Flotilla).
73 Gunkan Tsugaru sento shôhô (Detailed battle reports of the warship Tsugaru).
74 Nantô Hômen sakusen kiroku (Record of operations in the South-East Area).

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): 53–118.
Reference for this web page: http://ajrp.awm.gov.au/ajrp/ajrp2.nsf/translation/Chapter2?opendocument

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