'I' (ii) 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. Allied intelligence had detected the build-up of aircraft and the fighter sweep by 58 Mitsubishi A6M Reisen 'Zero' single-engined fighters was intercepted near the Russell islands group by 42 US fighters, mostly Grumman F4F Wildcat single-engined machines with a handful each of Lockheed P-38 Lightning twin-engined heavy fighters and Vought F4U Corsair single-engined fighters. The Japanese claimed to have shot down 47 US fighters (more than the number actually engaged), while the Americans claimed the destruction of 18 A6M fighters. The actual losses were nine A6M fighters and six US aircraft, with three of the US pilots recovered.
The first of the offensive’s main raids was 'Attack Operation X' flown against Guadalcanal on 7 April by 67 Aichi D3A 'Val' single-engined dive-bombers escorted by 110 Mitsubishi A6M Reisen 'Zero' single-engined fighters. The raid was intercepted by 76 Allied fighters (34 F4F, nine F4U, six Curtiss P-40, 12 Lockheed P-38 Lightning and 13 Bell P-39 Airacobra fighters) from Dobodura. At 14.00 Guadalcanal broadcast 'Condition Red' followed shortly by an unprecedented and highly irregular 'Condition Very Red'. The Japanese claimed to have shot down 41 US aircraft, and the Americans claimed the destruction of 26 A6M and 13 D3A machines, while the actual losses were 12 A6M and nine D3A aircraft for the Japanese and seven F3F aircraft for the Americans, with all the US plots recovered. A number of dive-bombers managed to leak through the Allied defences, however, and the Japanese pilots claimed 12 ships sunk and two heavily damaged. Actual Allied shipping losses were the US destroyer Aaron Ward, the 7,404-ton oiler Kanawha and the New Zealand corvette Moa, while one transport and one tanker were damaged.
The next raid took place on 11 April, when from 12.20 22 D3A dive-bombers and 72 A6M fighters attacked Oro Bay in the Buna area of New Guinea in 'Attack Operation Y'. The attack was met by 50 Allied fighters from Dobodura, and these shot down six Japanese aircraft without loss. However, a cargo ship was sunk, another crippled, and a minesweeper damaged.
Then on 12 April a force of 43 Mitsubishi G4M 'Betty' twin-engined medium bombers of the 705th Kokutai and 751st Kokutai and 131 A6M fighters of the 253rd Kokutai, the latter detached from the fleet carriers Zuikaku and Zuiho, reached and attacked Port Moresby. Opposed by 44 Allied fighters, briefly diverted by a Japanese feint toward Milne Bay and Allied radar glitches, most of the attackers were able to reach Port Moresby, but then failed to concentrate their attacks on the best targets, and succeeded only in damaging a few small craft and destroying four aircraft on the airfields. Ten Japanese aircraft were lost, either shot down or in crashes as they attempted to land back at their bases.
Finally, on 14 April, 188 aircraft attacked Milne Bay, over which they were intercepted by 24 Curtiss Kittyhawk Mk IA single-engined 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 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.
A halt to 'I' (ii) was ordered on 16 April when Yamamoto ordered the carrier air groups back to their carriers via the base at Truk in the Caroline islands group. Japanese claims to success in 'I' (ii) were greatly exaggerated: one cruiser, two destroyers, and 25 transport vessels sunk, and 175 aircraft shot down. The actual Allied losses were one destroyer, one tanker, one corvette and two cargo vessels sunk, and 25 aircraft lost. Though believing the operation to have been a major success, the Japanese were beginning to be alarmed by their own losses: 26% of the D3A dive-bombers and 18% of the G4M medium bombers had already been expended. Yamamoto’s decision to employ carrier air groups from land bases has been heavily criticised as a waste of the highly trained aircrew, whose carrier landing skills were irreplaceable. The actual results achieved hardly justified the diversion of so many resources, and the operation did not significantly delay Allied preparations for further offensives in the South and South-West Pacific Areas.
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 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 twin-engined 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.
It is worth noting that another series of Japanese raids began on 7 June 1943. On that day and on 12 June, Japanese aircraft raided Guadalcanal but again failed to achieve decisive results. On 16 June came a much larger raid by more than 100 aircraft. Two Allied ships were forced to beach and six Allied fighters were shot down, but the Allies claimed 98 Japanese aircraft destroyed, and the Japanese admitted the loss of about 30 aircraft.