Short answers, pulled from the story.
Operation Downfall was the proposed plan by United States and British Commonwealth forces for the invasion of the Japanese home islands near the end of World War II. It consisted of two parts, Operation Olympic and Operation Coronet.
Operation Olympic was scheduled to begin on X-Day, which was set for the 1st of November 1945. This operation aimed to capture the southern third of Kyūshū using Okinawa as a staging area.
Responsibility for planning fell to American commanders Fleet Admiral Chester Nimitz, General Douglas MacArthur, and the Joint Chiefs of Staff including Fleet Admirals Ernest King and William D. Leahy. Generals George Marshall and Henry H. Arnold also participated in the command structure.
Operation Coronet assigned up to 45 U.S. divisions for both initial landing and follow-up operations. The First Army and Eighth Army would comprise 25 divisions between them during the initial stage at Kujūkuri Beach and Hiratsuka.
The plan was canceled when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki, the Soviet declaration of war, and the invasion of Manchuria. No part of the operation ever took place because hostilities ended before the scheduled dates.