Operation Downfall was the proposed Allied invasion plan for Japan’s home islands, cancelled after Japan surrendered.
The plan had two major stages. Operation Olympic was intended to land on Kyushu in November 1945, while Operation Coronet was planned for the Kanto Plain near Tokyo in spring 1946. Okinawa was expected to serve as a staging area for the invasion.
Downfall never occurred because Japan surrendered after the atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war and invasion of Japanese-held territory. Allied planners expected the invasion to be exceptionally costly because Japan was preparing for a desperate defense of the home islands.
Operation Downfall is sometimes confused with Operation Overlord, the Allied invasion of German-occupied France in June 1944. Overlord was executed; Downfall remained a contingency plan. Had it occurred, it would have been among the largest amphibious operations in history.