Harry Truman succeeded Franklin D. Roosevelt as president before the end of the war. An unpretentious man who had previously served as Democratic senator from Missouri, then as vice president, Truman initially felt ill-prepared to govern the United States. Roosevelt had not confided in him about complex postwar issues and he had little prior experience in international affairs. "I'm not big enough for this job," he told a former colleague.

But Truman responded quickly to new challenges. Impulsive, he proved willing to make quick decisions about the problems he faced. A sign on his White House desk, since famous in American politics, read "The Buck Stops Here," and reflected his willingness to take responsibility for his actions. His judgments about how to respond to the Soviet Union had an important impact on the early Cold War.