On July 14th, 1945 American battleships South Dakota, Indiana, and Massachusetts bombarded Japan for the first time in preparation for an eventual invasion of the mainland. Operation Downfall was the name given to the planned invasion of Japan. The operation was to be divided into two parts –Operation Olympic and Operation Coronet. Operation Olympic was the sub-plan that targeted the Japanese home island of Kyushu. It would take place on 1 Nov 1945. The invasion fleet would escort 14 American divisions, both Army and Marine Corps, which were to conquer and hold the southern third of Kyushu.
Operation Coronet was to take place on 1 March 1946. It would have been the largest amphibious operation in history, with 25 divisions participating in the initial invasion. The invasion beaches were to be at Kujikuri on the Boso Peninsula and Hiratsuka at Sagami Bay, the forces were to work their way north across the Kanto plain toward Tokyo.
The main concern for the Americans was the potential for huge casualties. The Joint Chiefs of Staff estimated that Olympic alone would cost 456,000 men, including 109,000 killed. If the staff included Coronet estimates ran as high as 1.2 million casualties. Staff working for Admiral Nimitz calculated that in the first 30 days of Operation Olympic there would be the loss of 49,000 men. MacArthur’s staff estimated 125,000 casualties after 120 days of fighting. Personnel at the Navy Department estimated that the total losses would be to between 1.7 and 4 million with 400,000 to 800,000 deaths.
Fortunately Operation Downfall was never carried out, with America’s use of two atomic bombs and Russia’s sudden declaration of war on Japan, the war ended before such a costly invasion had to be initiated.