CIA Intelligence Monograph: The Final Months of the War with Japan: Signals Intelligence, U.S. Invasion Planing, and the A-Bomb decision. (1998).

The monograph notes the following points:

”By mid-1944, a consensus had begun to develop on the need at least to plan and prepare for an invasion, even though some officials evidently continued to believe there was a good chance it would not have to be carried out.” (That is, over a year before the war actually ended the military was considering the need for an invasion of the Japanese home islands themselves. Others in the military were of the belief that the Japanese would surrender due to their islands being surrounded by the US military and lack of supplies, including lack of food, for the civilians.)

The date for the invasion was set for November 1, 1945.

The invasion was going to take place on Kyushu, a fact which the Japanese either actually knew or had figured out. The Japanese busily mined their harbors and evacuated some of the civilians from the area as well as began to move large numbers of troops into the area.

”Intercepted communications also continued to reflect preparations for extensive use of suicide tactics. One series of messages indicated that up to 2,000 obsolete planes and trainers were being assigned to equip and train units for kamikaze missions. Instructions were issued to outfit biplanes and other older model aircraft with night operations equipment.”

”Other intercepted transmissions contained information on the construction of underground hangars and new, concealed dispersal airfields on Kyushu-which the analysts presumed would be used by suicide aircraft. A message in mid-June contained a Japanese naval base commander's description of progress being made on construction of suicide boats, and others dealt with measures to disperse and conceal unmanned 'boat bombs.' One intercepted order revealed the presence of a base for piloted suicide torpedoes on the southeastern tip of Kyushu.”

The report contains a table showing the ratio of US killed, wounded and MIA to the Japanese killed or taken prisoner (not counting wounded). In the battle of Leyte the ratio was 1:4.6 (For every American killed or wounded, 4.6 Japanese were killed or taken prisoner.) The ratio in Iwo Jima was 1:1.25, and the ratio in the battle for Okinawa was 1:2, showing the stiffening of Japanese resistance as the US got closer to their mainland area.

The early figuring on expected US casualties if Kyushu was invaded was around 105,050 during the first 90 days of battle. Later figures were higher, and some guesses predicted much higher casualties.

Further intercepted communications showed that there would be 940 suicide aircraft in 18 concealed bases on Kyushu, along with more bases for piloted torpedoes. The predictions for the level of buildup done earlier turned out to be wrong, as the Japanese were building up their forces on Kyushu faster than the US expected.

The strengthening of the Japanese forces was considered so important that apparently alternative plans for the invasion itself had begun to be considered, possibly along the lines of no actual invasion but continued bombing and mining, closing Japan off to the outside world. Later a plan for invading a different, less well-defended area of Japan was considered.

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