Operation Downfall was the name given to the planned invasion of Japan. Operation Downfall itself was divided into two parts - Operation Olympic and Operation Coronet. By mid-1945, it was apparent that the collapse of Japan was near and the Allies had to plan for the invasion of the Japanese mainland - something that they knew would be very costly in terms of lives lost.
American military commanders were given the task of planning for the invasion - Douglas MacArthur, Chester Nimitz, Ernest King, William Leahy, Hap Arnold and George Marshall. Inter-service rivalry did occur as both army and navy wanted one of 'their men' to be supreme commander of planning. Eventually the navy accepted that MacArthur was to have total control if the invasion was to take place. The planning proceeded without taking the atomic bomb into consideration as so few knew about its existence.
The Americans faced one very serious problem. They knew for sure that the Japanese would defend their territory with zeal and that American casualties would be high - probably too high for the American public to accept. The fanaticism that had been shown by the kamikazes, would almost certainly be encountered in Japan and the Americans had to plan for this.
There was plenty of evidence to indicate that any invasion of the Japanese mainland would be very bloody for all concerned. The complexity of such an attack also led to both sides of the US military developing different ideas as to what the best plan should be. The navy believed that a blockade supported by an air campaign would suffice. They wanted to use air bases in China and Korea to launch bombing raids against key cities in Japan. The army believed that such a campaign would take too long and that the morale of the American public might suffer as a result. They supported the use of an invasion that would go to the heart of Japan - Tokyo. The army got its way.
It quickly became apparent that any invasion of Japan would present huge difficulties. There were very few beaches that could be used as a landing place and the Japanese knew this. Both sides knew that only the beaches in Kyushu and the beaches at Kanto, near Tokyo, could support a huge amphibious landing. The Japanese took the appropriate measures in both areas.
The Americans had planned to land in Kyushu first and use it as a base for planes to attack other targets in Japan. These planes would then be used to give support to the landings at Kanto. As there were so few places to land a massive force of amphibious troops, the Japanese guessed as early as 1944 where such landings would take place.
The actual invasion of Kyushu was known to be fraught with dangers. Therefore, there were those in the American military who advocated the use of chemical weapons on the Japanese defenders. The use of poisonous gas had been outlawed by the Geneva Convention, but neither America nor Japan had signed this. As Japan had used poisonous gas in their attack on China, there were some in the US military who felt it was perfectly justified to use it on the Japanese. The Japanese did fear a gas attack and records do show that senior military figures in Japan wanted to ensure that if there was a gas attack, that the response of the Japanese would be such that it would not make any attack worse. American Intelligence had known for a while that Japan was in no fit state to respond to a gas attack with a gas attack.
The main concern for the Americans was the potential for huge casualty rates. Nearly every senior officer involved in the planning did his own research regarding American casualties - this was based on the experience America had fighting the Japanese since Pearl Harbour.
The Joint Chiefs of Staff estimated that Olympic alone would cost 456,000 men, including 109,000 killed. Including Coronet, it was estimated that America would experience 1.2 million casualties, with 267,000 deaths.
Staff working for Chester Nimitz, calculated that the first 30 days of Olympic alone would cost 49,000 men. MacArthur's staff concluded that America would suffer 125,000 casualties after 120 days, a figure that was later reduced to 105,000 casualties after his staff subtracted the men who when wounded could return to battle.
General Marshall, in conference with President Truman, estimated 31,000 in 30 days after landing in Kyushu. Admiral Leahy estimated that the invasion would cost 268,000 casualties. Personnel at the Navy Department estimated that the total losses to America would be between 1.7 and 4 million with 400,000 to 800,000 deaths. The same department estimated that there would be up to 10 million Japanese casualties. The 'Los Angeles Times' estimated that America would suffer up to 1 million casualties.
Regardless of which figures were used, it was an accepted fact that America would lose a very large number of men. This was one of the reasons why President Truman authorised the use of the atomic bomb in an effort to get Japan to surrender. On August 6th, 'Little Boy' was dropped on Hiroshima and on August 9th, 'Fat Man' was dropped on Nagasaki. On September 2nd, Japan surrendered and America and her allies were spared the task of invading Japan with the projected massive casualties this would entail.