Operation Coronet was tentatively scheduled to begin on March 1, 1946.
Coronet was the planned invasion of Honshu, Japan’s largest island and the location of Tokyo. It formed the second stage of Operation Downfall, following Operation Olympic’s proposed invasion of Kyushu. Military planners expected Coronet to be even larger than Olympic because it would require a drive across the Kantō Plain toward the capital.
The operation’s planned date was known as “Y-Day.” American forces expected to land near Kujūkuri Beach on the Bōsō Peninsula and at Hiratsuka on Sagami Bay. The plan envisioned as many as 45 U.S. divisions for the initial and follow-up phases, along with possible British Commonwealth support.
Coronet never occurred because Japan surrendered in August 1945. The planned invasion is therefore a counterfactual military operation, not a battle. Its projected size helps explain why the end of the war is discussed alongside invasion plans even though Allied troops did not need to fight their way into Tokyo.