'Tan 1' was a Japanese unrealised plan for an air attack on the US Navy’s fleet anchorage in the lagoon of Majuro atoll in the Marshall islands group (June 1944).
In February 1944 the Japanese naval general staff had started to develop a plan for a night attack on Admiral Raymond A. Spruance’s 5th Fleet’s anchorage at Majuro, this 'Yu' being posited on the departure of the Japanese navy’s 1st Mobile Fleet from the Inland Sea and attack Majuro with more than 500 carrierborne warplanes in conjunction with attacks by some 300 land-based warplanes. At the same time five submarines were to launch 10 amphibious light tanks equipped with torpedoes to attack the US carriers in the anchorage. Two other submarines were to land men of a special naval landing force to attack shore installations.
In March 1944, 'Yu' was approved by Admiral Mineichi Koga, the commander-in-chief of the Combined Fleet, and Admiral Shigetaro Shimada, the chief of the Japanese naval staff, but at the end of the same month Koga was killed when his flying boat crashed in bad weather en route from Palau to Davao. 'Yu' then went into abeyance pending the appointment of a new commander-in-chief of the Combined Fleet.
Meanwhile Commander Mitsuo Fuchida of Vice Admiral Kakuji Kakuta’s 1st Air Fleet, an officer who had made his name with the planning of the air attack that marked the culmination of 'Ai', was working on a plan for a smaller-scale attack on Majuro. This 'Tan 1' was based on the flying of a reconnaissance mission from Truk atoll to establish that there were worthwhile targets in the anchorage, and then an attack led by Fuchida with 27 torpedo bombers and, under the command of Lieutenant Commander Takashige Egusa, 27 dive-bombers.
'Tan 1' was dependent on reconnaissance, and Lieutenant Commander Takehiko Chihaya ordered that the 121st Naval Air Group despatch two three-seat Nakajima C6N1 Saiun 'Myrt' long-range reconnaissance aircraft from Tinian, in the Mariana islands group, on 3 June. These aircraft would refuel at Truk atoll and then fly to Nauru island to refuel again before starting the reconnaissance mission proper. Chihaya joined the two C6N1 aircraft on 4 June, and on the night of 4/5 June flew to Majuro, timing his flight to arrive at dawn on 5 June, the hour of the initial proposed attack. Finding eight fleet carriers and four escort carriers in the anchorage, Chihaya took photographs and flew back to Truk. After refuelling, he flew to Tinian to deliver the reconnaissance images to Fuchida. Chihaya was immediately instructed to fly another reconnaissance mission, but US air attacks on Truk prevented this. On 9 June 1944 Chihaya flew a final reconnaissance flight from Truk to Majuro, and then back to Nauru and Truk, but had to report that the anchorage was empty.
The 5th Fleet had departed Majuro for the 'Forager' invasion of Saipan in the Mariana islands group, and 'Tan 1' was thereupon cancelled.