The setbacks at the battles of the Coral Sea and Midway in May and June resulted in the postponement of the planned Japanese naval advance to Fiji, Samoa and New Caledonia (FS Operation). The Army Section of Japanese Imperial GHQ responded by ordering the 17th Army to study the feasibility of conducting an overland offensive against Port Moresby (Ri Operation Feasibility Study).

The South Seas Force, under the command of Major General HORII Tomitarô, had provided the main strength of the successful invasions at Guam in December 1941 and Rabaul in January 1942. Since the failure of the May sea-route offensive against Port Moresby, the South Seas Force had been attached to the 17th Army, and was subsequently ordered to undertake the overland offensive. The 1st Battalion of the 144th Infantry Regiment joined the 15th Independent Engineer Regiment and other support units to form the Yokoyama Advance Party.

The main concern for planning, despite the obvious deficiencies of materiel strength and air-support over the planned battlefield, was logistical: could a force be supplied and marched across the rugged mountains with sufficient strength at the end to defeat a growing Allied presence at Port Moresby? Major General HORII expressed to his superiors at the end of June that he felt the task would require too great an investment in manpower to transport the required food, munitions and other supplies unless a substantial part of the distance could be traversed by vehicle.

The geography of the proposed route from Buna on the north coast, through Kokoda in the rugged Owen Stanley Range and on to Port Moresby, was determined from local testimony, documents, maps, and aerial reconnaissance photos. The evidence available to Staff Officers of the 17th Army was inconclusive as to the existence and quality of roads over the mountains. Despite this, the misjudgement that a vehicular road could be available for supply from Buna to Kokoda had disastrous consequences later in the campaign.

The 17th Army sought cooperation from the 4th Fleet, who assigned Rear Admiral MATSUYAMA Kôji to command the 18th Flotilla (Tenryû and Tatsuta) and the 29th Destroyer Squadron (Asanagi, Yuzuki and Uzuki). The transportation of men, horses, vehicles and supplies from Rabaul to Buna would be undertaken by the high-speed transports Ryoyo Maru and Ayatozan Maru. Aerial protection for the transports and landings were provided by the 25th Air Flotilla and the Tainan Air Corps.

The main strength of the South Seas Force remained at Rabaul to prepare for transportation to Buna in mid-August. It was originally formed around the 144th Infantry Regiment and support units from the 55th Division. It had also acquired additional engineer, marine transport, communications and water supply units to provide specific services in the jungles and mountains of the Owen Stanley Range.

On 15 July, one week before the planned departure of the Advance Force, 17th Army Headquarters was advised to proceed with the overland offensive without waiting for the results of the feasibility study. Imperial Headquarter's Staff Officer TSUJI Masanobu was sent to Davao and then Rabaul to make arrangements for these operations. The determination to proceed with the offensive was made on his own authority, but was soon followed by detailed army orders.

The Yokoyama Advance Force, along with one company of the Sasebo Special Naval Landing Party and naval base unit troops, effected a successful landing in the Buna and Gona areas in the evening of 21 July. They met little resistance and soon pushed inland to prepare roads and supply lines for the imminent advance of the main strength of the invasion force.

Contributed by Steven Bullard (AJRP)

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