What was the Allied codename for the planned invasion of Japan that became unnecessary after surrender?

Operation Downfall was the Allied codename for the planned invasion of Japan.

The operation was designed as a massive two-stage assault on the Japanese home islands. Operation Olympic was scheduled to begin with an invasion of Kyushu in November 1945. Operation Coronet, aimed at the Tokyo Plain on Honshu, was planned for March 1946. Together, those operations formed Operation Downfall.

The plan was never carried out because Japan accepted the Allied surrender terms in August 1945. The atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, Japan's military situation, and internal political decisions all formed part of the final surrender context. Because Downfall was cancelled, its projected casualties remain estimates rather than recorded battlefield losses.

Operation Downfall is sometimes confused with Operation Overlord, the Allied invasion of Nazi-occupied France in June 1944. Overlord belonged to the European theater, while Downfall was the proposed final campaign against Japan in the Pacific.