What was the planned second major phase of Operation Downfall against Japan called?

The planned second major phase of Operation Downfall against Japan was called Operation Coronet.

Operation Downfall was the proposed Allied invasion of the Japanese home islands. Its first phase, Operation Olympic, was scheduled for November 1945 and would have attacked southern Kyūshū. Operation Coronet was planned as the follow-up invasion of the Kantō Plain near Tokyo, tentatively scheduled for March 1946.

Coronet was never carried out because Japan surrendered in August 1945. Allied planners expected it to be an enormous operation requiring large numbers of American troops, with additional British Commonwealth forces available as reinforcements.

Operation Coronet is sometimes confused with Operation Olympic, because both belonged to Downfall. Olympic targeted Kyūshū, while Coronet targeted the region around Tokyo on Honshu. Japanese surrender made both invasion plans unnecessary, so their projected battles and casualty totals remain counterfactual military history rather than actual campaigns.