On August 6, 1945, an American plane dropped an atomic bomb that detonated over Hiroshima, Japan—a target chosen because almost alone among major Japanese cities, it had not yet suffered damage. In an instant, nearly every building in the city was destroyed. Of the city’s population of 280,000 civilians and 40,000 soldiers, approximately 70,000 died immediately. Because atomic bombs release deadly radiation, the death toll kept rising in the months that followed. By the end of the year, it reached at least 140,000. Thousands more perished over the next five years. On August 9, the United States exploded a second bomb over Nagasaki, killing 70,000 persons. On the same day, the Soviet Union declared war on Japan and invaded Manchuria. Within a week, Japan surrendered.

Because of the enormous cost in civilian lives—more than twice America’s military fatalities in the entire Pacific war—the use of the bomb remains controversial. The Japanese had fought ferociously while being driven from one Pacific island after another. An American invasion of Japan, some advisers warned Truman, might cost as many as 250,000 American lives. No such invasion was planned, however, until 1946, and considerable evidence had accumulated that Japan was nearing surrender. Already some of its officials had communicated a willingness to end the war if Emperor Hirohito could remain on his throne. This fell short of the Allies’ demand for “unconditional surrender,” but the victors would, in the end, agree to Hirohito’s survival. Japan’s economy had been crippled and its fleet destroyed, and it would now have to fight the Soviet Union as well as the United States. Some of the scientists who had worked on the bomb urged Truman to demonstrate its power to international observers. But Truman did not hesitate. The bomb was a weapon, and weapons are created to be used.

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