What was the first phase of the planned Allied invasion of Japan under Operation Downfall called?

The first phase of the planned Allied invasion of Japan under Operation Downfall was called Operation Olympic.

Olympic was designed to land Allied forces in southern Kyūshū, Japan’s southernmost main island. The operation was scheduled to begin on 1 November 1945, with Okinawa intended to serve as a staging area for the invasion force.

The second phase, Operation Coronet, was planned for spring 1946 and would have targeted the Kantō Plain near Tokyo on Honshū. Together, Olympic and Coronet formed Operation Downfall, the overall proposed invasion of Japan’s home islands.

Japan surrendered before either phase occurred. The invasion plans were canceled after the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the Japanese government’s decision to accept Allied terms. Operation Olympic is therefore a planned operation, not a battle that actually took place.