Operation Downfall was the overall Allied plan for the invasion of Japan near the end of World War II. The operation was cancelled when Japan surrendered after the atomic bombing of Hiroshima and Nagasaki, and the Soviet Union's declaration of war against Japan.
Operation Downfall had two parts: Operation Olympic and Operation Coronet. Set to begin in October 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.
Later, in spring 1946, Operation Coronet was the planned invasion of the Kantō plain, near Tokyo, on the Japanese island of Honshū. Airbases on Kyūshū captured in Operation Olympic would allow land-based air support for Operation Coronet.
Japan's geography made this invasion plan obvious to the Japanese as well; they were able to predict accurately the Allied invasion plans and accordingly adjust their defensive plan, Operation Ketsugō. The Japanese planned an all-out defense of Kyūshū, with little left in reserve for any subsequent defense operations.
Casualty predictions varied widely but were extremely high for both sides: depending on the degree to which Japanese civilians resisted the invasion, estimates ran into the millions for Allied casualties[1] and tens of millions for Japanese casualties.