This was the Japanese offensive designed to eliminate the rapidly growing naval and air power of the Allied powers, and especially the USA, in the south-eastern Pacific with a sustained air offensive launched from the north-western end of the Solomon islands chain and even from the larger airfields of their base areas round Rabaul in New Britain and Kavieng in New Ireland (7/16 April 1943).
A mere two months after the US seizure of the Russell islands group in 'Cleanslate' only weeks after the Japanese 'Ke' (i) evacuation of their last forces from Guadalcanal, the Japanese hoped with this offensive against Allied ships and air bases to cause significant damage, but more importantly to entice US fighter strength into the air and defeat it, so delaying the impending offensive by the forces of Admiral William F. Halsey’s South Pacific Area command up the chain of the Solomon islands group toward the islands of New Georgia ('Toenails') and Bougainville ('Cherryblossom').
To boost the 250-aircraft strength of Vice Admiral Jinichi Kusaka’s 11th Air Fleet, the Japanese called into this theatre most of the depleted carrier air strength (about 100 aircraft of the 1st Carrier Squadron) from Admiral Isoroku Yamamoto’s Combined Fleet at Truk atoll in the Caroline islands group). These were to join the aircraft of the 11th Air Fleet in operations from the land bases around Rabaul to attack Allied ships, aircraft and land installations in the south-eastern part of the Solomon islands group and in New Guinea. The operation’s objective was to halt the development of Allied offensives in New Guinea and the Solomon islands group, and at the same time give Japan time to prepare a new series of defensive positions in response to recent defeats by the Allies in the Guadalcanal campaign and in New Guinea at Buna-Gona, Wau and the Bismarck Sea.
The nature of the offensive and the distance of the bases around Rabaul entailed operations over very protracted ranges, with the result that mechanical problems often turned into insuperable difficulties on the long homeward trips, especially for aircraft which had suffered combat damage, causing the Japanese to lose heavily against US air forces growing almost daily in strength and tactical capability.
The offensive proper was prefaced by an air sweep down ‘The Slot’ on 1 April by 58 Japanese fighters, of which 18 were downed for the loss of six US fighters. The first of the offensive’s main raids was flown against Guadalcanal on 7 April by 67 Aichi D3A 'Val' dive-bombers escorted by 110 Mitsubishi A6M Reisen 'Zero' fighters. The raid was intercepted by 76 Allied fighters, and the air battle resulted in the loss of 21 Japanese and seven Allied aircraft. The attack also sank the US destroyer Aaron Ward, 7,404-ton oiler Kanawha and New Zealand corvette Moa, as well as damaging one transport and one tanker.
The next raid took place on 11 April, when 22 D3A bombers and 72 A6M fighters attacked Oro Bay in the Buna area of New Guinea. A total of 50 Allied fighters scrambled from Dobodura and intercepted the force, shooting down six Japanese aircraft without loss to themselves.
Then on 12 April a force of 43 Mitsubihi G4M 'Betty' medium bombers of the 705th Kokutai and 751st Kokutai and 131 A6M fighters of the 253rd Kokutai detached from the fleet carriers Zuikaku and Zuiho reached and attacked Port Moresby. Opposed by 44 Allied fighters, the attack resulted in the loss of two Allied and five Japanese aircraft, and succeeded in damaging only a few small craft as well as damaging and destroying a number of aircraft on Port Moresby’s airfields.
Finally, on 14 April, 188 aircraft attacked Milne Bay, over which they were intercepted by 24 Curtiss Kittyhawk Mk IA fighters of the Royal Australian Air Force: the air battle which followed cost the Japanese seven aircraft and the Australians three machines. The 2,996-ton Dutch cargo ship van Heemskerk was hit by several bombs, and the ship was beached while still burning. The 3,533-ton British cargo ship Gorgon was also hit by several bombs and was set on fire, but the blaze was later extinguished. Near misses damaged the 2,069-ton Dutch cargo ship van Outhoorn and the Australian minesweepers Wagga and Kapunda. Four Allied servicemen and 12 merchant seaman were killed in the air raid, and another 68 men were injured.
Although the Japanese had sunk several Allied transports and warships, the offensive failed to inflict serious damage on the Allied forces. Based on inaccurate and unintentionally exaggerated reports from the aircrews involved, Yamamoto halted the attacks on 16 April, believing the operation to have been a success. The operation did not significantly delay Allied preparations for further offensives in the South Pacific Area, however.
In overall terms the raids cost the Allies some 25 aircraft but, more importantly, they also resulted in further irreplaceable losses of skilled aircrew among Japan’s carrierborne air groups. When ‘I’ (ii) was ended on 16 April, the much depleted air groups returned to Truk.
Another consequence of the offensive’s failure was Yamamoto’s death. The Japanese admiral decided on a morale-boosting visit to outlying Japanese garrisons after the failure of ‘I’ (ii), and this fact became known to the Americans as they had broken the Japanese naval code, and therefore planned ‘Vengeance’. On 18 April, the two Mitsubishi G4M bombers carrying Yamamoto and his staff from Rabaul to Kahili on the southern tip of Bougainville island were intercepted and shot down by 16 Lockheed P-38 Lightning long-range fighters of the USAAF operating at the very limit of their endurance from Henderson Field on Guadalcanal island. Command of the Combined Fleet then passed to the more cautious Admiral Mineichi Koga.