Late in the afternoon of April 12, 1945, Presidential Press Secretary Steve Early summoned Harry Truman to the White House. Expecting to see President Franklin Roosevelt, he was ushered instead into the study of Eleanor Roosevelt. “Harry,” she informed him, “the President is dead.” Momentarily stunned, Truman eventually spoke and asked with genuine concern, “Is there anything I can do for you?” Mrs. Roosevelt replied insightfully: “Is there anything we can do for you for you are the one in trouble now.” Within two hours Truman recited the oath of office, becoming the thirty-third president of the United States. Immediately after his swearing-in ceremony the new president addressed the hastily convened cabinet. “It was my intention,” Truman recalled saying, “to continue both the foreign and domestic policies of the Roosevelt Administration.” He sincerely attempted to fulfill this pledge. Understanding how he did so, and appreciating the new president's basic convictions and his broad approach on foreign policy, are essential for apprehending the place that the atomic bomb played in his formulation of it.

Truman's comprehension of the details of Roosevelt's foreign policy and his grasp of the contemporary international situation were distinctly limited, to put it mildly. But it is important to appreciate that he was not some sort of human tabula rasa on these matters. His arrival in the United States Senate in 1935 forced him to devote some sustained – if, at times, naive – attention to defense and foreign policy questions.