Harry S. Truman (May 8 1884 – December 26 1972) was the thirty-third President of the United States (1945–1953). As vice president, he succeeded Franklin D. Roosevelt, who died less than three months after he began his fourth term.

During World War I Truman served as an artillery officer. After the war he became part of the political machine of Tom Pendergast and was elected a county judge in Missouri and eventually a United States Senator. After he gained national prominence as head of the wartime Truman Committee, Truman replaced vice president Henry A. Wallace as Roosevelt's running mate in 1944.

As president, Truman faced challenge after challenge in domestic affairs. The tumultuous reconversion of the economy of the United States was marked by severe shortages, numerous strikes, and the passage of the Taft-Hartley Act over his veto. He confounded all predictions to win re-election in 1948, largely due to his famous Whistle Stop Tour of rural America. After his re-election he was able to pass only one of the proposals in his Fair Deal program. He used executive orders to begin desegregation of the U.S. armed forces and to launch a system of loyalty checks to remove thousands of communist sympathizers from government office, even though he strongly opposed mandatory loyalty oaths for governmental employees, a stance that led to charges that his administration was soft on communism. Truman's presidency was also eventful in foreign affairs, with the end of World War II and his decision to use nuclear weapons against Japan, the founding of the United Nations, the Marshall Plan to rebuild Europe, the Truman Doctrine to contain communism, the beginning of the Cold War, the creation of NATO, and the Korean War. Corruption in Truman's administration reached the cabinet and senior White House staff. Republicans made corruption a central issue in the 1952 campaign.

Truman, whose demeanor was very different from that of the patrician Roosevelt, was a folksy, unassuming president. He popularized such phrases as "The buck stops here" and "If you can't stand the heat, you better get out of the kitchen." He overcame the low expectations of many political observers who compared him unfavorably with his highly regarded predecessor. At one point in his second term, near the end of the Korean War, Truman's public opinion ratings reached the lowest of any United States president, but popular and scholarly assessments of his presidency became more positive after his retirement from politics and the publication of his memoirs. He died in 1972. Many U.S. scholars today rank him among the top ten presidents. Truman's legendary upset victory in 1948 over Thomas E. Dewey is routinely invoked by underdog presidential candidates.