Operation Downfall was the planned Allied invasion of Japan’s home islands if Japan refused to surrender in 1945.
The plan had two major stages. Operation Olympic was scheduled to begin in November 1945 with an invasion of southern Kyushu. Operation Coronet was planned for spring 1946 and would have targeted the Kantō Plain near Tokyo on Honshu. Okinawa was intended to serve as a major staging area.
Downfall was canceled after Japan accepted the Potsdam Declaration and the war ended. The invasion was expected to be extraordinarily costly because Japanese forces prepared for an all-out defense of the home islands, while civilians were also being organized for resistance. Casualty estimates varied widely, so no single figure can be treated as definitive.