Operation Downfall was the overall Allied plan for the invasion of Japan during World War II, . This plan had its genesis in 1943 and the “Strategic Plan for the Defeat of Japan.” It envisioned two subordinate attacks in 1946 - Olympic, the invasion of the southern island of Kyushu, and Coronet, the attack of the main island of Honshu. These invasions would have dwarfed the Normandy invasion in terms of the number of troops involved, the difficulty of logistics and projected casualties. This operation was canceled as a result of the surrender of Japan.
Operation Olympic, the invasion of Kyushu, was to begin November 1, 1945. Fifteen divisions were scheduled to take part in this landing. The naval armada would have been the largest ever assembled, including forty-two aircraft carriers, twenty-four battleships and almost four hundred destroyers and destroyer escorts. Using Okinawa as a staging base, Olympic was to seize the southern portion of Kyushu. It was also to include a deception plan, known as Operation Pastel. See also Ten Ichi and Majestic.
Because of a known buildup of Japanese troups on Kyushu leading up to the invasion, some American military planners proposed the use of chemical weapons prior to the invasion. It would not have been a violation of the Geneva convention, which prohibits first use of them; Japan had earlier used them against the Chinese in Manchuria.
Operation Coronet, the invasion of Honshu at the Tokyo Plain south of the capital, was set to begin on December 1, 1945. This was later postponed to March, 1946. Coronet would have been the largest amphibious operation of all time, with fifteen divisions (including the floating reserve) earmarked for the operation.