Harry S. Truman entered Missouri politics after military service in the First World War and an unsuccessful retail business. Elected to Jackson County administrative office, he worked within the Democratic organization associated with Tom Pendergast. As a United States senator from 1935, he gained recognition by leading the committee investigating defense production, exposing waste and helping improve wartime procurement. Selected as Franklin Roosevelt's running mate in 1944, he became vice president in January 1945.

Roosevelt's death on April 12, 1945, brought Truman to the presidency during the final phase of the Second World War. He participated in the Potsdam Conference and approved the use of atomic bombs against Hiroshima and Nagasaki. The bombings contributed to Japan's surrender while causing devastating civilian deaths; their necessity and alternatives remain subjects of historical debate. Truman also supported the creation of the United Nations and confronted the difficult transition from wartime mobilization to a peacetime economy.

As relations with the Soviet Union deteriorated, his administration pursued containment. The Truman Doctrine offered assistance to Greece and Turkey in 1947, while the Marshall Plan supported European recovery. The Berlin airlift answered the Soviet blockade, and the formation of NATO in 1949 institutionalized an Atlantic alliance. He recognized Israel in 1948.

At home Truman proposed the Fair Deal, but Congress rejected much of its program. He issued orders against discrimination in federal employment and for equality of treatment in the armed forces in 1948, important steps toward military desegregation. His civil rights position divided Democrats, yet he defeated Thomas Dewey in that year's election through an energetic campaign. Labor disputes and the federal loyalty program also exposed tensions between his reform ambitions, economic pressures, and concern about communist influence.

After North Korea invaded South Korea in 1950, Truman committed American forces under United Nations authority. Chinese intervention turned an initially successful counteroffensive into a prolonged war. He dismissed General Douglas MacArthur in 1951 after disagreement over strategy and public challenges to administration policy, asserting civilian control. The conflict and other controversies weakened his popularity. He left office in January 1953 and devoted retirement to memoirs and his presidential library. He died in 1972.