What was the code name of the planned Allied invasion of Japan’s main island of Honshu?

The planned Allied invasion of Japan’s main island of Honshu was code-named Operation Coronet.

Operation Coronet was intended to be the second major phase of Operation Downfall, the proposed invasion of the Japanese home islands. Its target area was the Kantō Plain near Tokyo, and planners expected the operation to begin in March 1946 if Japan had not surrendered.

The first phase, Operation Olympic, was planned for Kyūshū in November 1945. Olympic was intended to establish bases and secure southern Kyūshū before the much larger Coronet assault. The two operations were part of a broader plan involving enormous numbers of American and Allied troops.

Japan’s surrender in August 1945 canceled both phases before they began. This is why Operation Coronet is sometimes mistakenly described as an actual battle or invasion. It remained a contingency plan rather than a completed operation. Operation Downfall was the umbrella name, while Olympic and Coronet were its two proposed component operations.