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Operation 'I' or 'I-Go' (7-16 April 1943) was the Japanese Navy's attempt to compensate for the loss of Guadalcanal by launching a series of massive aerial assaults on the American's new advanced bases.
When the Japanese military leaders informed Emperor Hirohito of their decision to evacuate Guadalcanal, he asked them what they were planning to do next. Their answer was that they intended to stop the enemies move westwards.
The army planned to do this by moving 100,000 men onto New Guinea, but this moved ended in disaster when the first major troop convoy was destroyed by the Americans (battle of the Bismarck Sea). The large scale movement was cancelled.
The navy decided to carry out a series of massive aerial strikes at one of the new US bases - Port Moresby and Milne Bay on New Guinea or Guadalcanal. On 25 March they decided to make the main effort against the New Guinea bases, and placed Admiral Yamamoto in charge of the operation.
On 3 April Yamamoto flew from Truk to Rabaul to take direct control of the operation. He found 86 fighters, 27 dive-bombers and 72 twin-engined bombers available from the land based Eleventh Air Fleet. To them he added 96 fighters, 65 dive-bombers and a small number of dive bombers from the Third Fleet carriers Zuikaku, Zuiho, Junyo and Hiyo to give him just under 182 fighters, 97 dive bombers and 72 twin-engined bombers. This air fleet was concentrated at Rabaul, and the fighters were then moved to bases in the upper Solomons.
Although Yamamoto had been ordered to focus his efforts on Papua, he chose to launch the first attack against the shipping off Guadalcanal. A preliminary fighter sweep on 1 April didn't go well, with eighteen of fifty eight Zeros lost and only six American fighters shot down.
Operation 'I' began in earnest on 7 April, when Yamamoto sent 67 Aichi D3A 'Val' dive bombers and 110 Mitsubishi Zeros to attack the Allied ships in Savo Sound, north of Guadalcanal, in the larger Japanese air attack since Pearl Harbor. Japanese reconnaissance had reported that there were four cruisers, eight destroyers and fourteen transport ships in the sound at 11.00, but by the time the Japanese aircraft arrived the three cruisers actually present and six destroyers had already left.
The Americans had some advance warning of the attack. Their reconnaissance aircraft spotted the concentration of Japanese aircraft at Kahili and Ballale, and at 12.26 a coastwatcher reported the first aircraft as they left Buka. Other coastwatchers reported the departure of other parts of the Japanese aerial armada and the Americans were able to scramble 76 fighters from Henderson Field in time for them to have reached altitude before the Japanese arrived.
The fighting began at around 3pm. The American fighters had the best of the clash with the Zeroes, but the Vals were allowed to pass by and attack the shipping. Their first victim was the oil tanker Kanawha, which was attacked by fifteen aircraft, hit several times and set on fire. The captain ordered his crew to abandon ship. The fires were later put out and she was beached on Tulagi, but overnight she slipped back into deep water and sank. Nineteen men were lost.
The New Zealand corvette HMNZS Moa was hit by two bombs that were actually aimed at an oiler she was refuelling from. The Moa sank in four minutes, but only five men were lost.
The third and final loss was the destroyer USS Aaron Ward. She was preparing to engage aircraft that had been sighted to the north when three Vals attacked from the sun. The engine room was destroyed and the fire rooms flooded. The ship remained afloat, and an attempt was made to tow her to Tulagi, but she sank three miles from safety.
In the air the Americans lost seven US Marine fighters, although six of the pilots survived. The Japanese lost twelve Vals and nine Zeros to a mix of US fighters and anti-aircraft fire. For the next ten days the Americans at Guadalcanal prepared for a repeat attack, but it never came. After a successful first strike Yamamoto's attention now turned towards Papua.
The first strike came on 11 April when 22 Vals and 72 Zeros attacked shipping in Oro Bay, south-east of Buna on New Guinea. Once again the Japanese achieve some success, sinking one merchant ship and forcing another to beach. This time the air battle was more one sided, with six Japanese aircraft lost at no cost to the Americans.
12 April saw the biggest air raid yet carried out in the South-west Pacific. This time Yamamoto sent 131 fighters and 43 medium bombers to attack shipping that he believed to be present at Port Moresby. Forty four Allied fighters rose to intercept. They were unable to stop the medium bombers, but no ships damaged and the Allies once again came out on top in the air, losing two aircraft to five Japanese, despite being badly outnumbered.
On 14 April Milne Bay was the target. Once again the Japanese increased the size of their attack force, this time to 188. The bay was defended by 24 RAAF Kittyhawks, and contained a number of potentially valuable targets. At 11.00am when the incoming raid had been detected the bay contained four or five corvettes, three Dutch troop transports (van Heemskerk, van Outhoorn and Balikpapam) and the British motor ship Gorgon. Commander Branson of the Royal Australian Navy used the advanced warning well. The troops were unloaded, and the troop ships ordered to escape to Waga Waga. The corvettes and the Gorgon were ordered to make to sea.
Branson's efforts almost certainly averted heavy losses. The first wave of Japanese aircraft, of high level bombers, attacked the anchorage just after noon and hit the areas that had only just been evacuated.
The Japanese dive bombers arrived at about 12.30 and were a little more effective. The Gordon was hit twice, and all but five of her crew were killed. The ship itself was towed to shore and saved. The Van Heemskerk was hit and suffered fatal damage. Once again the Allies came out better in the air, losing three aircraft and shooting down seven.
By now the Japanese believed that they had won a major victory, and had sunk one cruiser, two destroyers, twenty five transports and shot down 175 aircraft. The real losses were one destroyer, one corvette, one oiler, two merchant ships and around 25 aircraft. Admiral Yamamoto believed the exaggerated claims of his men, and on 16 April he officially ended Operation 'I'. The Japanese had lost 39 aircraft and believed that they had won a major victory.
The failure of Operation 'I' was compounded on 18 April. The Americans had broken many Japanese codes, and now learnt that Yamamoto was planning to inspect new air bases at Buka and Buin. This put him within range of P-38s from Henderson Field. Just after 9.30 on 18 April, while approaching Buin, Yamamoto's aircraft was shot down and he was killed.
The failure of Operation 'I' and the death of Yamamoto were a double blow for the Japanese, although they were only aware of the second. The Japanese believed that they had inflicted heavy losses on the Americans and must have set back any offensive plans they had. The loss of Yamamoto was a blow both to Japanese morale and to the quality of their leadership.