Operation Coronet was planned to invade the Kantō Plain during the proposed Allied invasion of Japan. The operation was intended to follow Operation Olympic and strike the broad plain surrounding Tokyo on Honshu.
The Kantō Plain was a logical objective because it contained Tokyo, major transportation networks, industrial centers, and a large share of Japan’s population. Capturing it would have placed the Japanese government and the country’s political heart under direct Allied pressure.
Coronet was projected for spring 1946, after Olympic had established bases on Kyūshū. Japan’s surrender in August 1945 made both operations unnecessary. The proposed invasion is often discussed under the overall name Operation Downfall, but Coronet was specifically the Honshu phase, not the Kyūshū landings. That distinction helps separate the two planned operations.