Because the U.S. military planners assumed "that operations in this area will be opposed not only by the available organized military forces of the Empire, but also by a fanatically hostile population", high casualties were thought to be inevitable, but nobody knew with certainty how high. Several people made estimates, but they varied widely in numbers, assumptions, and purposes — which included advocating for and against the invasion — afterwards, they were reused to debate over the atomic bombings of Hiroshima and Nagasaki.
Casualty estimates were based on the experience of the preceding campaigns, drawing different lessons:
In a study done by the Joint Chiefs of Staff in April, the figures of 7.45 casualties/1,000 man-days and 1.78 fatalities/1,000 man-days were developed. This implied that a 90-day Olympic campaign would cost 456,000 casualties, including 109,000 dead or missing. If Coronet took another 90 days, the combined cost would be 1,200,000 casualties, with 267,000 fatalities.
A study done by Adm. Nimitz's staff in May estimated 49,000 casualties in the first 30 days, including 5,000 at sea. A study done by General MacArthur's staff in June estimated 23,000 in the first 30 days and 125,000 after 120 days.When these figures were questioned by General Marshall, MacArthur submitted a revised estimate of 105,000, in part by deducting wounded men able to return to duty.
In a conference with President Truman on June 18, Marshall, taking the Battle of Luzon as the best model for Olympic, thought the Americans would suffer 31,000 casualties in the first 30 days (and ultimately 20% of Japanese casualties, which implied a total of 70,000 casualties). Adm. Leahy, more impressed by the Battle of Okinawa, thought the American forces would suffer a 35% casualty rate (implying an ultimate toll of 268,000). Admiral King thought that casualties in the first 30 days would fall between Luzon and Okinawa, i.e., between 31,000 and 41,000.
Of these estimates, only Nimitz's included losses of the forces at sea, though kamikazes had inflicted 1.78 fatalities per kamikaze pilot in the Battle of Okinawa, and troop transports off Kyūshū would have been much more exposed.
A study done for Secretary of War Henry Stimson's staff by William Shockley estimated that conquering Japan would cost 1.7 to 4 million American casualties, including 400,000 to 800,000 fatalities, and five to ten million Japanese fatalities. The key assumption was large-scale participation by civilians in the defense of Japan.
Outside the government, well-informed civilians were also making guesses. Kyle Palmer, war correspondent for the Los Angeles Times, said half a million to a million Americans would die by the end of the war. Herbert Hoover, in memorandums submitted to Truman and Stimson, also estimated 500,000 to 1,000,000 fatalities, and were believed to be conservative estimates; but it is not known if Hoover discussed these specific figures in his meetings with Truman. The chief of the Army Operations division thought them "entirely too high" under "our present plan of campaign."
The Battle of Okinawa, the very last pitched battle against Japan, ran up 72,000 casualties in 82 days, of whom 18,900 were killed or missing. (This is conservative, because it excludes several thousand U.S. soldiers who died after the battle indirectly from their wounds.) The entire island of Okinawa is 464 square miles; to take it, therefore, cost the United States 407 soldiers (killed or missing) for every 10 square miles of island. If the U.S. casualty rate during the invasion of Japan had only been 5 percent as high per square mile as it was at Okinawa, the United States would still have lost 297,000 soldiers (killed or missing).
Nearly 500,000 Purple Heart medals were manufactured in anticipation of the casualties resulting from the invasion of Japan. To the present date, all the American military casualties of the sixty years following the end of World War II — including the Korean and Vietnam Wars — have not exceeded that number. In 2003, there were still 120,000 of these Purple Heart medals in stock. There are so many in surplus that combat units in Iraq and Afghanistan are able to keep Purple Hearts on-hand for immediate award to wounded soldiers on the field.
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