What was the planned Allied invasion of Japan's main islands called?

The planned Allied invasion of Japan’s main islands was called Operation Downfall.

Downfall was the proposed U.S. and British Empire plan to invade the Japanese home islands if Japan did not surrender. It had two major stages: Operation Olympic, aimed at southern Kyushu, and Operation Coronet, aimed at the Kanto Plain near Tokyo.

Olympic was scheduled for November 1945, while Coronet was planned for spring 1946. Okinawa was intended to serve as a staging area and air-support base for the invasion. Allied planners expected fierce resistance because Japanese forces were preparing for a decisive defense of the homeland.

Downfall was canceled after Japan surrendered following the atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war on Japan. Had it occurred, it would have been the largest amphibious operation in history, surpassing the Normandy landings. Casualty estimates varied widely and remain a subject of historical debate.