Operation Downfall was the codename for the planned but never executed American invasion plan for the Japanese mainland islands during the end stages of World War II in the Pacific Theatre. It was set to occur in October of 1945 and if it had taken place it would have been the largest amphibious attack in human history. The estimated American casualties for the invasion ranged wildly from one hundred thousand American casualties to over one million. The estimated numbers were calculated based on earlier battles in Iwo Jima and Okinawa and the heavy death tolls to American soldiers in these battles, and the expectation that the Americans would not only be fighting the Japanese Army but also a hostile civilian population who sought to protect their homeland.
Due to the large number of expected casualties and the development of the atomic bomb, United States President Harry S. Truman made the decision to American made atomic bombs to end the war in the Pacific. The atomic bombing of Hiroshima and Nagasaki in 1945 would go on to be to one of the most important and memorable events in all of human history.