Operation Downfall was the proposed Allied plan for the invasion of the Japanese home islands near the end of World War II. The planned operation was canceled when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the invasion of Manchuria. The operation had two parts: Operation Olympic and Operation Coronet. Set to begin in November 1945, Operation Olympic was intended to capture the southern third of the southernmost main Japanese island, Kyūshū, with the recently captured island of Okinawa to be used as a staging area. In early 1946 would come Operation Coronet, the planned invasion of the Kantō Plain, near Tokyo, on the main Japanese island of Honshu. Airbases on Kyūshū captured in Operation Olympic would allow land-based air support for Operation Coronet. If Downfall had taken place, it would have been the largest amphibious operation in history, surpassing D-Day.

The surrender of the Empire of Japan in World War II was announced by Emperor Hirohito on 15 August and formally signed on 2 September 1945, bringing the war's hostilities to a close. By the end of July 1945, the Imperial Japanese Navy (IJN) had become incapable of conducting major operations and an Allied invasion of Japan was imminent. Together with the United Kingdom and China, the United States called for the unconditional surrender of the Japanese armed forces in the Potsdam Declaration on 26 July 1945—the alternative being "prompt and utter destruction". While publicly stating their intent to fight on to the bitter end, Japan's leaders were privately making entreaties to the publicly neutral Soviet Union to mediate peace on terms more favorable to the Japanese. While maintaining a sufficient level of diplomatic engagement with the Japanese to give them the impression they might be willing to mediate, the Soviets were covertly preparing to attack Japanese forces in Manchuria and Korea in fulfillment of promises they had secretly made to the United States and the United Kingdom at the Tehran and Yalta Conferences.