Truman became president after the death of Roosevelt, taking his place as one of the 'Big Three' at the Potsdam Conference. He felt that Roosevelt had been too soft on Communism, and therefore took a tougher line towards Stalin. His leadership revealed deep tensions between the USA and the USSR.

In August 1945, he authorised the first atomic bomb to be dropped on Hiroshima, believing that it would rapidly put an end to war with Japan. Some suggest that he did so to demonstrate his military superiority over the USSR. Truman saw France and Italy as particularly vulnerable to a Communist takeover.

In a speech to the US Congress in 1947, he adopted the 'Truman Doctrine', a policy aimed at containing the advance of Communism at any cost, anywhere in the world. This Policy of containment informed much of his hard-line Cold War policy towards the USSR.