This was the Japanese strategic defence plan created by the Imperial General Headquarters in Tokyo for the strategic conduct of all Japanese operations from that time onward in a great arc running from Manchukuo (the Japanese puppet state in Manchuria) through the Pacific to the Southern Region (24 July 1944).
The battles of 1943 had expelled the Japanese army from its bases in the south-eastern end of the Solomon islands group and the Gilbert islands group, and in 1944 a series of US amphibious landings supported by potent carrierborne air strength had culminated in the seizure of the major parts of the strategically vital Mariana islands group. The US victory in the Battle of the Philippine Sea resulting from 'A' during June had destroyed Japanese carrierborne airpower and established Allied air and sea superiority over the western part of the Pacific Ocean. This gave the Allies freedom to choose where to strike next.
Admiral Chester W. Nimitz, commanding the Pacific Ocean Areas, favoured a blockade of the Japanese forces in the Philippine islands group and an attack on Formosa as possession of the latter would give the Allies control of the sea routes between Japan and southern and south-eastern Asia, severing Japan’s links with its primary resources area and also isolating Japanese garrisons, which would then perish from lack of supplies.
General Douglas MacArthur, commanding the South-West Pacific Area, favoured an invasion of the Philippine islands group, which also lay across the supply lines to Japan. Leaving the Philippine islands group in Japanese possession would be a blow to US prestige and a personal affront to MacArthur, who in 1942 had famously vowed to return.
President Franklin D. Roosevelt was called in to adjudicate the dispute, and came down in favour of MacArthur’s option.
The Allied options were equally apparent to the Japanese navy. Admiral Soemu Toyoda, heading the Combined Fleet, made essentially the same appraisal of the strategic situation and reached basically the same conclusions as the Americans, and in response prepared four ‘Sho’ plans, namely ‘Sho 1’ as a major naval operation in response to any US approach to or landing on the Philippine islands group, and ‘Sho 2’, ‘Sho 3’ and ‘Sho 4’ as responses to any attacks on Formosa, the Ryukyu islands group and the Kurile islands group respectively. The plans envisaged typically complex undertakings that committed all available Japanese forces to a decisive battle.
As noted above, the plan was prepared in the aftermath of the Japanese defeats in the Battle of the Philippine Sea and on the island of Saipan (soon to be joined by Tinian and Guam) in the Mariana islands group, and the beginning of US air raids on the Japanese home islands. These events had led to the fall on 18 July of the administration headed by General Hideki Tojo, to be replaced on 22 July by a government led by General Kuniaki Koiso and Admiral Mitsumasa Yonai. The evident fact that the Japanese planners had to face was that their enemy (in this instance the Americans) were overwhelmingly stronger than the Japanese in the air, on land and at sea, and that the US offensive against Japan would have to be met and defeated in the area of the Philippine islands group, Formosa and Japan. Here the defence would enjoy the benefits of interior (and hence shorter) lines of communication, and of lengthy preparation.
The basic ‘Sho’ plan therefore provided for a defensive concentration on the line running from the south in the Philippines via Formosa, the Ryukyu islands and the Japanese home islands to the Kurile islands in the north, and this basic geographical layout created the framework for the ‘Sho’ plan.
In parallel with this Pacific planning, ‘Ichi’ was to be pursued in China with the objects of denying the USAAF the bases from which they could bomb Japan, of securing the Chinese harvest and of inflicting on the Chinese a decisive defeat so that Japanese ground and air forces could be redeployed to theatres that were becoming a greater threat to Japan.
In the region of Field Marshal Count Hisaichi Terauchi’s Southern Expeditionary Army Group, the defence was a joint army and navy responsibility by area, but in September the two services agreed on a combined defence for Celebes (General Korechika Anami’s 2nd Area Army with six infantry and one air divisions), Borneo (Lieutenant General Masataka Yamawaki’s 37th Army with a miscellany of garrison units), the Andaman islands group and the Nicobar islands group (Major General Yoshisuke Inoue’s 35th Independent Mixed Brigade and Major General Toshio Itsuki’s 36th Independent Mixed Brigade respectively), and in October the army assumed sole responsibility for the defence of these island groups.