The Cold War was a state of geopolitical tension after World War II between powers in the Eastern Bloc (the Soviet Union and its satellite states) and powers in the Western Bloc (the United States, its NATO allies and others). Historians do not fully agree on the dates, but a common timeframe is the period (the second half of the 20th century) between 1947, the year the Truman Doctrine (a U.S. foreign policy pledging to aid nations threatened by Soviet expansionism) was announced, and 1991, the year the Soviet Union collapsed.

The term "cold" is used because there was no large-scale fighting directly between the two sides, although there were major regional wars, known as proxy wars, supported by the two sides. The Cold War split the temporary wartime alliance against Nazi Germany, leaving the Soviet Union and the United States as two superpowers with profound economic and political differences. The USSR was a Marxist–Leninist state ruled by its Communist Party and secret police, who in turn were ruled by a dictator (Stalin) or a small committee ("Politburo"). The Party controlled the press, the military, the economy and all organizations. It also controlled the other states in the Eastern Bloc, and funded Communist parties around the world, sometimes in competition with Communist China, particularly following the Sino-Soviet split of the 1960s. In opposition stood the West, dominantly democratic and capitalist with a free press and independent organizations. A small neutral bloc arose with the Non-Aligned Movement; it sought good relations with both sides. The two superpowers never engaged directly in full-scale armed combat, but they were heavily armed in preparation for a possible all-out nuclear world war. Each side had a nuclear strategy that discouraged an attack by the other side, on the basis that such an attack would lead to the total destruction of the attacker: the doctrine of mutually assured destruction (MAD). Aside from the development of the two sides' nuclear arsenals, and their deployment of conventional military forces, the struggle for dominance was expressed via proxy wars around the globe, psychological warfare, massive propaganda campaigns and espionage, rivalry at sports events, and technological competitions such as the Space Race.