When Harry Truman succeeded Franklin Roosevelt as president in April 1945, the United States had just begun the systematic, low-level saturation bombing of Japanese cities. In the third month of his administration, the new president received word of the nuclear test at Alamogordo, thought immediately of biblical prophesies of the apocalypse, and immediately approved the use of the atomic bombs against Japan. As he phrased it in his belatedly discovered “Potsdam diary,” written at the time he learned about the successful test, the Japanese were “savages, ruthless, merciless and fanatic.” In a personal letter written a few days after Hiroshima and Nagasaki had been destroyed, the president explained that “when you have to deal with a beast you have to treat him as a beast.” Following Japan's capitulation in mid-August 1945, the United States occupied the country as the overwhelmingly dominant force in a nominally “Allied” occupation and proceeded to initiate a rigorous policy of “demilitarization and democratization.”

Less than five years later, the Truman administration had identified Japan as the key to the balance of power in Asia—and Asia as capable of tipping the global balance in the direction of the Soviet Union.