What was the codename for the planned Allied invasion of Japan as a whole?

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

Downfall was a proposed two-stage campaign intended to force Japan’s surrender through a massive invasion of the Japanese home islands. Its first phase, Operation Olympic, called for landings on southern Kyushu. Its second phase, Operation Coronet, was planned for the Kanto Plain near Tokyo on Honshu.

The invasion was never carried out because Japan surrendered in August 1945. Allied planners expected fierce resistance and prepared enormous forces, shipping requirements, and casualty estimates. The planned operation therefore provides important context for understanding why Japan’s surrender was viewed as a major turning point by Allied military leaders.

Operation Downfall is often confused with Olympic, but the names referred to different levels of the plan. Downfall was the overall invasion concept; Olympic concerned Kyushu, while Coronet concerned the later assault on Honshu. The atomic bombings and Soviet entry into the war changed the strategic situation before Downfall could begin.