This was a Japanese unrealised plan for an air attack on major elements of Admiral Chester W. Nimitz’s US Pacific Fleet anchored in Ulithi atoll in the Caroline islands group by aircraft-carrying submarines (August 1945).
The Japanese had established a weather and radio station at the atoll, and began the construction of an airfield and seaplane base on Falalop islet, but these were abandoned in the later part of 1944 in a process which also saw the removal of most of the native population to Yap.
Nimitz, commanding both the Pacific Fleet and the Pacific Ocean Areas, had noticed Ulithi atoll on a map, quickly realised its strategic potential, and arranged for its seizure on 23 September 1944 in the course of the Palau islands campaign. At this point there was still unwarranted optimism about the progress of 'Stalemate II' on Peleliu, and the 323rd Regimental Combat Team of Major General Paul J. Mueller’s 81st Division was detached from the Peleliu undertaking and detailed for the occupation of Ulithi. Cover was provided by a naval task group under the command of Rear Admiral William H. P. Blandy. There was no resistance, and unloading was completed in two days. A battalion of 'Seabee' construction troops followed, and Ulithi became the forward fleet base for the remainder of the war. A 1,200-ft (365-m) airstrip was constructed on Falalop islet and a hospital and fleet recreation centre were established on other larger islets.
Because the base was not seen as permanent, no oil tank farm was constructed. Instead, the US Navy maintained six to eight obsolete tankers at the anchorage each with a capacity of between 8,000 and 11,000 tons) to serve as a floating tank farm. A fleet of 40 tankers shuttled the oil from Ulithi to the fighting fleet.
The 'Arashi' plan involved two ‘I-400’ class submarines, each carrying three Aichi M6A1 Seiran catapult-launched attack warplanes, of Captain Ryunosuke Ariizumi’s 1st Submarine Flotilla within Vice Admiral Chuichi Hara’s 4th Fleet, and was schemed on the basis of targeting information provided by ‘Hikari’.
The plan was two days from implementation at the time of the Japanese surrender.