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

The planned Allied invasion of Japan’s main island Honshu was called Operation Coronet.

Coronet was the second stage of Operation Downfall, the proposed Allied invasion of the Japanese home islands. Planners intended it to begin in spring 1946, after Operation Olympic had captured the southern part of Kyushu. Coronet would have attacked the Kantō Plain near Tokyo, using large forces advancing from landing areas on the Bōsō Peninsula and Sagami Bay.

Operation Olympic was the separate first stage of Downfall and was aimed at southern Kyushu. Operation Ketsugō, by contrast, was Japan’s defensive plan for resisting an Allied invasion. These similarly named plans are often confused because they all belonged to the same projected campaign.

Japan surrendered before either Olympic or Coronet could begin. The proposed invasion was canceled after the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war on Japan, and Japan’s acceptance of surrender terms. Coronet was tentatively scheduled for March 1, 1946.