What was the Allied code name for the planned invasion of Japan if it had not surrendered?

Operation Downfall was the Allied code name for the planned invasion of Japan.

The proposed operation was divided into two major phases. Operation Olympic was intended to land on southern Kyushu in November 1945, while Operation Coronet was planned for the Tokyo plain in March 1946. Together, these operations formed Downfall.

The invasion plans were developed because Allied leaders expected Japan might continue fighting after the atomic bombings and the Soviet entry into the war. Military planners anticipated severe resistance from Japanese forces and civilians, along with substantial Allied and Japanese casualties.

Japan’s acceptance of the Potsdam Declaration made the invasion unnecessary. The formal surrender ceremony took place on 2 September 1945, before Olympic could begin. Operation Downfall is therefore remembered as a planned campaign rather than a battle. Its projected scale helps explain why the surrender decision was treated as a major turning point by Allied governments.