'Kikusui' (ii) was the Japanese ultimate kamikaze mission by the super-battleship Yamato in the Battle of the East China Sea (6/7 April 1945).

By the spring of 1945 Admiral Soemu Toyoda’s once formidable Combined Fleet had been trimmed to just a handful of operational warships and a few remaining obsolescent aircraft manned by semi-trained aircrew. Most of the Combined Fleet's surviving warships were stationed at ports in Japan, with most of the large ships at Kure, near Hiroshima, on the northern side of the Inland Sea. With the 'Forager' and 'Detachment' invasions of the Mariana island group and Iwo Jima, the US forces began their campaign against the Japanese homeland itself. As the next step before a planned invasion of the Japanese mainland, US forces launched the 'Iceberg' invasion of Okinawa on 1 April 1945.

During March, in the course of a briefing about Japan’s response to the expected invasion of Okinawa, Japanese military leaders explained that the Japanese army was planning extensive air attacks, including the use of kamikaze aircraft. Emperor Hirohito then asked what the Japanese navy was planning and, piqued by this possible slur, the navy planned a kamikaze mission for its remaining major assets, which included Yamato.

Drafted under Toyoda’s supervision, the resulting plan envisaged that Yamato and her escorts would steam to Okinawa, attack the US fleet supporting the land forces in the island, and then beach themselves between Higashi and Yomitan and then operate as shore batteries until destroyed, whereupon any surviving members of the crews were to join the island’s defenders. Very little air cover, if any at all, could be provided for the ships, which would therefore be all but helpless in the face of concentrated US air attacks.

In preparation for the implementation of this plan, the assigned ships left Kure for Tokuyama, off Mitajiri, on 29 March. However, in spite of obeying orders to prepare for the mission, Vice Admiral Seiichi Ito, commanding the undertaking, refused to order his ships to carry it out, believing the plan to be futile and wasteful. Vice Admiral Ryunosuke Kusaka flew to Tokuyama from Tokyo on April 5 in a final attempt to convince the assembled commanders of the Combined Fleet, including Ito, to accept the plan. Upon first hearing of the proposed operation, the Combined Fleet's senior officers unanimously joined Ito in rejecting it for the same reasons that he had expressed. Kusaka then explained that the Japanese navy’s attack would help divert US aircraft away from the Japanese army’s planned air attacks on the US fleet at Okinawa. He also explained that Japan’s national leadership was expecting the Japanese navy to make a major effort to support the defence of Okinawa.

The Combined Fleet's senior officers now acceded to the proposed plan. The ships' crews were briefed on the operation and, though offered the opportunity to remain behind, no one did so, though the crews' newer, sick and infirm members were ordered off the ships. By the middle of the night the ships had been fuelled and, apparently and in defiance of orders to provide the ships with only just enough fuel to reach Okinawa, the base personnel at Tokuyama gave the ships almost all of the fuel remaining in the port, although this was probably still not enough to get the ships back to Japan.

At 16.00 on 6 April Yamato, light cruiser Yahagi and destroyers Isokaze, Hamakaze and Yukikaze of the 17^th Destroyer Division, Asashimo, Kasumi and Hatsushimo of the 21^st Destroyer Division, and Fuyutsuki and Suzutsuki of the 41^st Destroyer Division departed Tokuyama.

The US submarines Threadfin and Hackleback sighted the Japanese squadron as it proceeded to the south through the Bungo Suido, could not attack, but notified the US fleet of the Japanese sortie. At dawn on 7 April, the Japanese force passed the Osumi peninsula into the open ocean heading to the south from Kyushu in the direction of Okinawa. The ships steamed in defensive formation at 20 kt, with Yahagi leading Yamato and the eight destroyers deployed in a ring around the two larger ships, each ship some 1,640 yards (1500 m) from its neighbour. It was at about this time that the destroyer Asashimo developed engine problems and turned back.

US aircraft were now shadowing the squadron, and at 10.00 the Japanese ships turned to the west to give the impression that they were withdrawing, but at 11.30 they turned back toward Okinawa. At about 10.00 on the same day, the US Navy had started to launch almost 400 Grumman F6F Hellcat fighters, Curtiss SB2C Helldiver dive-bombers, and Grumman TBF Avenger torpedo bombers in several waves from the carriers of Vice Admiral Marc A. Mitscher’s 1^st Fast Carrier Task Force (Task Force 58) 1 operating just to the east of Okinawa.

The US plan was to intercept and destroy Yamato's squadron with carrierborne aircraft, and only if the air attacks proved unsuccessful to engage Japanese ships with the heavy guns of the battleships.

As the Japanese squadron had no air cover, the US aircraft were able to plan and implement their attacks without fear of Japanese aerial interference. Thus the US aircraft, reaching the Japanese squadron after a two-hour flight, were able to circle the Japanese formation just out of anti-aircraft range, methodically setting up their attacks on the warships below.

The first wave of carrierborne aircraft engaged the Japanese at 12.30. The ships increased to 25 kt, began evasive manoeuvring, and opened fire with their anti-aircraft guns. The torpedo bombers attacked mainly from the port side so that the concentration of damage on just one side would increase the likelihood of the target ship capsizing. At 12.46 a torpedo hit Yahagi directly in her engine room, killing the entire engineering room crew and bringing the cruiser to a complete stop. Yahagi was then struck by at least six more torpedoes and 12 bombs delivered by following waves of aircraft. The destroyer Isokaze attempted to aid Yahagi, but was herself attacked, heavily damaged and later sank. Yahagi capsized and sank at 14.05. Her survivors, left floating in the water, could see Yamato in the distance, still apparently steaming south and attempting to fight off the attacking aircraft.

During the first attack, and despite intensive evasive manoeuvring which caused most of the bombs and torpedoes aimed at her to miss, Yamato was struck by two armour-piercing bombs and one torpedo. Her speed was not affected, but one of the bombs started a fire abaft the superstructure. During the first wave of attacks the destroyers Hamakaze and Suzutsuki were heavily damaged and knocked out of the battle, Hamakaze later sinking.

Between 13.20 and 14.15 the second and third waves of US aircraft attacked, the weight of their efforts being concentrated on Yamato. During this time, the super-battleship was hit by at least eight torpedoes and up to 15 bombs. The bombs did extensive damage to the topside of the ship, knocking out power to the gun directors and forcing the anti-aircraft guns into local control with consequent degradation of their overall effectiveness. The torpedo hits, almost all on the port side, caused considerable flooding, to the extent that Yamato listed far enough that capsizing was now an imminent danger. The flooding damage-control station had been destroyed by a bomb hit, which made it impossible to counter-flood the specially designed spaces within the ship’s hull to counteract hull damage. At 13.33, in a desperate attempt to keep the ship from capsizing, Yamato's damage-control team counterflooded both starboard engine and boiler rooms. This mitigated the danger but also drowned the several hundred members of the crew. The deaths of those crew members bought Yamato about 30 more minutes of survival.

The loss of the starboard engines, plus the weight of the water, reduced Yamato's speed to about 10 kt. With their main target now moving more slowly, and therefore easier to target, US torpedo aircraft concentrated on hitting the super-battleship’s rudder and stern with torpedoes in order to affect her steering ability. These efforts were successful, and at 14.02, after being informed that the ship could no longer steer and was unavoidably sinking, Ito ordered the mission to be aborted, the crew to abandon ship, and the remaining ships to begin rescuing survivors. Yamato communicated this message to the other surviving ships by signal flag as her radio equipment had been destroyed.

By 14.05 Yamato was dead in the water and began to capsize. Ito and Yamato's captain decided to go down with the ship and refused to abandon her, and at 14.20 Yamato capsized completely and began to sink. At 14.23 she suddenly blew up with an explosion so large that it was reportedly heard and seen 125 miles (200 km) away in Kagoshima, and sent a mushroom-shaped cloud almost 20,000 ft (6095 m) into the air. The explosion is believed to have happened as the fires reached the main ammunition magazines.

Attempting to make it back to port, the destroyer Asashimo was bombed and sunk with all hands, the destroyer Kasumi was also sunk but, despite the fact that her bow had been blown off, Suzutsuki was able to make it to Sasebo in Japan by steaming in reverse the entire way. The three remaining less-damaged destroyers (Fuyuzuki, Hatsushimo and Yukikaze) were able to rescue 280 survivors of Yamato's crew of 2,700, as well as 555 survivors from Yahagi's crew of 1,000, and slightly more than 800 survivors from Hamakaze, Isokaze and Kasumi. However, 3,700 Japanese naval personnel perished in the battle.

Just 10 US aircraft were shot down by anti-aircraft fire from the Japanese ships, and some of their crews were rescued by amphibious aircraft or submarines. In total, the US Navy lost 12 men.

During the battle, aircraft of the Japanese army attacked the US fleet at Okinawa as promised, but failed to sink any ships. Around 115 aircraft, many of them kamikaze machines, attacked the US ships throughout 7 April. Kamikaze aircraft hit the fleet carrier Hancock, battleship Maryland and destroyer Bennett, causing moderate damage to Hancock and Maryland, and heavy damage to Bennett. About 100 Japanese aircraft were lost in these attacks.

This had been the last major Japanese naval operation of the war, and the surviving Japanese warships had little involvement in combat operations for the rest of the conflict. Suzutsuki was not repaired, and while Fuyuzuki was repaired, she hit an air-laid US mine at Moji in Japan on 20 August and was not subsequently repaired. Yukikaze survived the war almost undamaged. Hatsushimo hit an air-laid US mine on 30 July near Maizuru in Japan, and thus became the 129^th, and last, Japanese destroyer sunk in the war.