'Yu' was Japanese unrealised plan for an air and surface attack on the US Pacific Fleet’s major warships, especially the fleet and light carriers of the Fast Carrier Force, in the lagoon of Majuro atoll in the southern part of the Marshall islands group (May 1944).

The US forces had taken the island in January 1944 in 'Flintlock', and together with those of Eniwetok and Kwajalein in the same islands group, its lagoon was being developed into an advanced fleet base. The fact of these developments became apparent to the Japanese as early as February 1944, when long-range reconnaissance aircraft, flying primarily from Nauru and Ocean islands, which had been taken in 'Ry', revealed the work. The Japanese already had models on which to base the concept of such an attack, most notably their own 'Ai' operation against Pearl Harbor in December 1941, but also a raid undertaken by Lieutenant Colonel Evans F. Carlson’s 2^nd Marine Raider Battalion. On 17 August 1942, this unit landed from two submarines on Butaritari island, part of the Makin atoll system, and subsequently inflicted heavy damage, in the process compelling the Japanese to divert troops from the reinforcement of Guadalcanal. The Japanese had planned to raid Espíritu Santo in similar fashion in October 1942, but the idea was abandoned after submarine reconnaissance revealed the island to be too heavily held.

The Japanese attack on Majuro was first scheduled for 10/11 April but was later postponed. The basic scheme of 'Yu' was taking form on the basis of a schedule in which, early in May, the nine aircraft carriers and their screening warships of Vice Admiral Kiyohide Shima’s 1^st Mobile Force, 5^th Fleet, as well as four auxiliary carriers, with a total of 530 aircraft embarked, would depart the Inland Sea and pass the Bonin islands group, Marcus island and Wake island to reach a point to the north-east of Majuro, and then on the night of 20/21 May launch a surprise attack. This date was selected as it the moon would be nearly full, and therefore provide good light. At the same time, 300 land-based aircraft would be shuttled into the area through Marcus, Wake, Truk and Ponape islands to assist the carrierborne aircraft. Lastly, five submarines would offload amphibious light tanks, equipped to carry and launch aerial torpedoes, outside the atoll’s coral reef so that the tanks could cross the reef using their tracks and then launch their weapons at the carriers in the anchorage. A submarine picket line would protect the eastern flank of the 1^st Mobile Force, 5^th Fleet on its final approach to the target area.

The plan was developed under the leadership of Captain Chikao Yamamoto, chief of the operations branch of the Naval General Staff, and approved by Admiral Shigetaro Shimada, chief of the naval staff. Accompanied by a small staff, Yamamoto left Yokohama during the last week of March and flew to the island of Palau in the western part of the Caroline islands group, advance base for the Combined Fleet, for a conference with Admiral Mineichi Koga, its commander. The meeting took place on the night of 26/27 March 1944 on the super-battleship Musashi. As the plan was outlined to Koga and several of his staff, some voiced serious misgivings and suggested that the operation should be deferred until further studies could be completed. Yamamoto persisted and won the support of the majority of the staff, including Koga. The balance of the night was spent refining the basic outline. The night of 20/21 May was selected as the time for the attack and the plan, as previously presented to Shimada, was adopted.

The next morning Yamamoto and his staff left Musashi, went ashore, boarded their aeroplane and had returned to Tokyo by 29 March. Two days later Koga and his staff left Palau by flying boat, but were lost when the flying boat crashed. 'Yu' was thereupon shelved pending the appointment of a new commander of the Combined Fleet. For a month, the Combined Fleet was in the hands of a caretaker commander pending the arrival of Admiral Soemu Toyoda as Koga’s successor, and 'Yu' proceeded no further.

The core of 'Yu' as planned was the reinforced 1^st Mobile Fleet, 5^th Fleet and land-based bombers of the Japanese navy air force. The aircraft allocated to 'Yu' were to have been 60 land-based fighters, 40 land-based bombers, 100 carrierborne fighters, 50 carrierborne attack aircraft and two land-based reconnaissance aircraft of the 1^st Base Air Force together with 20 land-based fighters of the 22^nd Air Flotilla, a total of 272 aircraft. This was little more than half the number discussed in Yamamoto’s planning session.

The submarine part of 'Yu' was 'Tatsumaki' (tornado), and the submarines at the centre of this part of the plan’s components were I-36, I-38, I-41, I-44 and I-53, each carrying two Type 4 'Ka-Tsu' amphibious carrier vehicles on its after deck. The Type 4 was equipped to carry and launch 450-mm (17.7-in) Type 2 surface-launched torpedoes. On 26 March 1944, I-36 left Kure for a preliminary reconnaissance of Majuro, and this was undertaken on 22 April, the submarine’s Yokosuka E14Y1 'Glen' floatplane reporting the presence of 11 carriers and three battleships.

On 26 April, Vice Admiral Chuichi Nagumo assumed control of 'Yu', but expressed his opposition to a scheme which he deemed impossible of implementation. On 3 May Toyoda stated that the operation should be implemented regardless of cost, and this approval was echoed by that of the Imperial General Staff on the grounds that the operation’s successful implementation could become a 'turning point of the whole war'.

Early in May, Vice Admiral Takeo Takagi, commanding the 6^th Fleet, personally supervised the preparations for 'Yu' from the 2,417-ton submarine tender Tsukushi Maru. He concentrated five submarines and 14 tracked carriers at a small island off Katsurajima Channel in the Inland Sea. Several support vessels and a 30-ton floating crane also arrived from Kure to facilitate the loading of the tracked carriers onto the submarines. Five submarines, each carrying one tank apiece, practised their part in the operation. Initially optimistic about the whole plan, Takagi rapidly grew disillusioned. Commander Chojuro Takahashi, commanding the 15^th Submarine Division, was sure that the plan could not succeed, and therefore suggested full-scale landing trials by a submarine carrying two tracked carriers. The tests revealed that the tanks were extremely noisy, very slow in the water, and tended to shed their tracks if even the smallest obstacle was encountered. It was also discovered that their engine compartments were not entirely watertight and also that their covers tended to leak when their parent submarines were under the water, eventually flooding the carrier’s entire engine section.

On 12 May, after Takagi had submitted a negative report, 'Yu' was postponed 'for the time being', that is until the deficiencies of the Type 4 tracked carrier had been remedied.