'Kon' (i) was the Japanese strategic plan to reinforce and hold an inner defence line in the Pacific theatre (May/June 1944).
By the second quarter of 1944, the Japanese had come to appreciate the fact that two major US thrusts were being directed against them, namely that of Admiral Chester W. Nimitz’s Pacific Ocean Areas from east to west across the central Pacific, and that of General Douglas MacArthur’s South-West Pacific Area from south to north. The Japanese also appreciated that these two thrusts could well meet in the Philippine islands group. The defensive line for 'Kon' (i) thus ran from the Mariana islands group via the Palau islands group and western New Guinea to the Netherlands East Indies, and behind this line the Japanese built up troop concentrations, prepared new airfields and gathered Admiral Soemu Toyoda’s Combined Fleet.
The overall concept was to create the capability for the rapid movement of men, aircraft and warships to any sector threatened by the Americans, though the main thrust was nonetheless expected to be that of MacArthur’s forces from New Guinea toward Mindanao island in the Philippine islands group, where the Japanese thought that they could fight a decisive battle against the Americans with a high chance of success.
The actual concentrations and contingency plans were frequently altered to reflect shifts in the perceived threat. By May 1944 it had started to become clear that the strategic premises upon which 'Kon' (i) had been based were now out of date, and by mid-June 1944 Japanese thinking was changing to 'A' as the decisive battle against the advancing US forces.
The only element of 'Kon' (i) which was launched was thus an attempted reinforcement of Biak island after the US 'Horlicks' landing of 2 July 1944. The Imperial General Headquarters was determined not to permit any initial breach of its inner defence line, and the Japanese navy ordered about half of its land-based air strength from the central Pacific to concentrate on bases in or on western New Guinea. At the same time, the Japanese navy persuaded the Japanese army to contribute Major General Yoshio Tamada’s 2nd Amphibious Brigade for a reinforcement effort. This effort started with the loading of some 700 men at Zamboanga on 31 May in a convoy escorted by Rear Admiral Naomasa Sakonju’s 16th Cruiser Division (heavy cruiser Aoba and light cruiser Kinu), and destroyers Shikinami, Uranami and Shigure, with cover provided by a force from Tawi-Tawi comprising the battleship Fuso, heavy cruisers Myoko and Haguro, and five destroyers, the convoy proceeded to Davao. A third force, comprising the minelayers Itsukushima and Tsugaru and the tank landing ship T-127, was to move another 800 troops to Biak.
On 2 June the forces put to sea, but were located by a Consolidated Liberator patrol bomber and turned back on 3 June as Japanese reconnaissance aircraft had wrongly reported the presence of a US aircraft carrier in the region. On the same day aircraft of Rear Admiral Kenzo Ito’s 23rd Air Flotilla tried to attack US ships, but the destroyers Reid, Mustin and Russell beat off the attackers.
A second attempt to bring 600 troops to Biak was then made by Sakonju with the destroyers Shikinami, Uranami and Shigure, escorted by the destroyers Harusame, Shiratsuyu and Samidare. Cover was provided by the heavy cruiser Aoba and light cruiser Kinu near Vogelkop.
The Japanese attempt was again spotted by US reconnaissance aircraft, and Vice Admiral Thomas C. Kinkaid ordered the relevant detachments of his US 7th Fleet not to attack until the Japanese had reached Biak. US air reconnaissance once again located the Japanese force, which was then engaged by North American B-25 Mitchell medium bombers of Lieutenant General George C. Kenney’s 5th AAF, which sank one destroyer, but the other ships pressed forward to Biak, where they encountered Rear Admiral V. A. C. Crutchley’s force comprising the Australian heavy cruiser Australia, US light cruisers Phoenix and Boise and the US Destroyer Divisions 42, 47 and 48 (Fletcher, Jenkins, Radford, La Valette, Hutchins, Daly, Beale, Bache, Abner Read, Mullany, Trathen, and Australian Arunta and Warramunga). In an engagement in which the Allied destroyer force tried to catch the Japanese as they retired, Shiratsuyu was damaged but escaped. The US submarine Hake sank the destroyer Kazegumo off Davao on 8 June, and Harusame was sunk by US bombers on the same day.
A third Japanese attempt to deliver reinforcements to Biak was abandoned en route as the US forces launched 'Forager' against the Mariana islands group on 12 June, whereupon the ships received orders to take part in 'A'.
This third attempt had comprised two transport groups (Group A under Sakonju with the cruisers Aoba and Kinu and destroyers Shikinami, Uranami, Yamagumo and Nowake, and Group B with the minelayers Itsukushima and Tsugaru, a tank landing ship, freighters and submarine chasers) covered by Vice Admiral Matome Ugaki’s task force comprising the super-battleships Yamato and Musashi, heavy cruisers Myoko and Haguro, light cruiser Noshiro, and destroyers Shimakaze, Okinami and Asagumo, which were to have overwhelmed the Allied cruiser and destroyer force operating in support of 'Horlicks'.