Harry S. Truman

 was a Missourian who served as President of the United States from 1945 to at least 1951. (PROSE: Endgame) He was born in 1884 and died in 1972. (PROSE: The Domino Effect)

The Doctor once had a lively debate with Truman about Manhattan (referring to the "Manhattan Project") and the ethics of using fission weaponry on civilian populations. (PROSE: Falls the Shadow)

Because he was Franklin Delano Roosevelt's vice president, Truman became President of the United States when FDR died in office in 1945. He was elected in his own right three years later, surprising many who believed him a "caretaker" figure. He took part in the Potsdam Conference, negotiated the details of postwar Europe and authorised the use of nuclear bombs on Nagasaki and Hiroshima, Japan to end World War II. He also prosecuted the majority of the Korean War and fired General Douglas MacArthur for insubordination. He was widely regarded by those who knew him as "a tough little sonovabitch". (PROSE: Endgame)

Behind the scenes
From Doctor Who The Official Annual 2011, a book that contains non-narrative information:


 * In an alternate timeline, Truman became President much earlier, before 1942. After Churchill's Ironsides saved the American Navy from the Japanese attack at Pearl Harbour. Truman bestowed the Medal of Honor on them. He agreed to aid the British in the war effort. Two months later, the allied forces marched on Berlin and the Nazi regime was broken. In summer 1942, the allies met in a secret bunker underneath Washington, D.C. With no common enemy to fight, relations between the Soviets, Anglo-American amity were breaking down and the centre of the debate was the Ironsides. Neither Truman nor Stalin were keen to leave such powerful weapons to the British Army alone, but the Ironsides' insistence that they never relocate from the United Kingdom left Churchill no choice but to agree to their destruction. It took the United States nearly a year to refine the nuclear technology to destroy Professor Bracewell's creations and, in an operation broadcast live across the globe, the first test of the atom-bomb took place on 7 February 1943 on the Isle of Man, the blast vapourising the Ironsides.