On a warm summer morning above Hiroshima, Japan, an atomic bomb was for the first time dropped on a target as an act of war. The date was August 6, 1945. Less than a minute after the bomb was released, Hiroshima nearly disappeared in a brilliant cloud of atomic dust. The nuclear age had begun with a terrible fury. Sixty-six thousand people died and thousands more were injured; additional thousands later succumbed to radiation poisoning. Four square miles of the city were instantly erased.

President Harry S. Truman had given the order for a U.S. B-29 aircraft to deliver the bomb knowing what destruction it would cause. He later admitted that it weighed heavily on his conscience, but Truman felt he had no choice. In mid-June 1945, the administration was weighing the possibility of Japan's surrender. The Japanese navy had ceased to exist and Japan's military losses were...