What was the Allied plan for invading Japan in 1945 if Japan did not surrender?

The planned Allied invasion of Japan in 1945 was called Operation Downfall.

Operation Downfall was the overall plan for a massive invasion of the Japanese home islands. It had two main phases: Operation Olympic, the proposed invasion of southern Kyushu, and Operation Coronet, the planned attack on the Tokyo Plain on Honshu. The first phase was scheduled for November 1945, with the second planned for March 1946.

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 Union’s entry into the war against Japan, and Japan’s internal decision to surrender all preceded the projected invasion dates.

Operation Downfall is sometimes mistakenly used as the name of the Kyushu landing alone. Olympic was the Kyushu phase, while Downfall referred to the complete two-stage invasion plan. Estimates of potential casualties varied widely, and later claims about exact numbers often present disputed projections as if they were certain forecasts.