The Cold War was the state of conflict, tension and competition that existed between the United States and the Soviet Union and their respective allies from the mid-1940s to the early 1990s. Throughout this period, rivalry between the two superpowers was expressed through military coalitions, propaganda, espionage, weapons development, industrial advances, and competitive technological development, which included the space race.

300px-Reagan and Gorbachev hold discussions

The US and the Soviet Union were the two superpowers during the Cold War, each leading its own sphere of influence. Here, the respective leaders Ronald Reagan (left) and Mikhail Gorbachev meet in 1985.

Both superpowers engaged in costly defense spending, a massive conventional and nuclear arms race, and numerous proxy wars.

Although the US and the Soviet Union were allied against the Axis powers during World War II, the two states disagreed sharply both during and after the conflict on many topics, particularly over the shape of the post-war world. The war had either exhausted or eliminated the pre-war "Great Powers" leaving the US and USSR as clear economic, technological and political superpowers. In this bipolar world, countries were prompted to align themselves with one or the other of the superpower blocs (a Non-Aligned Movement would emerge later, during the 1960s).

The suppressed rivalry during the war quickly became aggravated first in Europe, then in every region of the world, as the US sought the "containment" and "rollback" of communism and forged myriad alliances to this end, particularly in Western Europe and the Middle East. Meanwhile, the Soviet Union fostered Communist revolutionary movements around the world, particularly in Eastern Europe, Latin America and Southeast Asia.

The Cold War saw periods of both heightened tension and relative calm. On the one hand, international crises such as the Berlin Blockade (1948–1949), the Korean War (1950–1953), the Berlin Crisis of 1961, the Vietnam War (1959–1975), the Soviet war in Afghanistan (1979–1989), and especially the 1962 Cuban Missile Crisis, raised fears of a Third World War. The last such crisis moment occurred during NATO exercises in November 1983, but there were also periods of reduced tension as both sides sought détente. Direct military attacks on adversaries were deterred by the potential for mutual assured destruction using deliverable nuclear weapons.

The Cold War drew to a close in the late 1980s and the early 1990s. With the coming to office of US President Ronald Reagan, the US increased diplomatic, military, and economic pressure on the Soviet Union, which was already suffering from severe economic stagnation. In the second half of the 1980s, newly appointed Soviet leader Mikhail Gorbachev introduced the perestroika and glasnost reforms. The Soviet Union collapsed in 1991, leaving the United States as the sole superpower in a unipolar world.