What was the codename for the proposed Allied invasion of Japan's main islands?

The proposed Allied invasion of Japan's main islands was codenamed Operation Downfall.

Operation Downfall was the overall plan for a large-scale invasion if Japan did not surrender. It had two major stages: Operation Olympic, the planned invasion of southern Kyushu, and Operation Coronet, the proposed assault on the Kantō Plain near Tokyo on Honshu. Olympic was scheduled for November 1945, while Coronet was planned for March 1946.

The expected casualties were a central concern for Allied planners. Japanese military and civilian preparations suggested that an invasion could be extremely costly, although estimates varied widely and no invasion took place. The atomic bombings, the Soviet entry into the war, Japan's internal political crisis, and the surrender decision made the operation unnecessary.

Operation Downfall is sometimes confused with Olympic because Olympic was the first phase rather than the name of the entire plan. Neither Downfall nor its component operations was carried out.