The Cold War was the period of conflict, tension and competition between the United States and the Soviet Union and their respective allies from the mid-1940s until the early 1990s. Throughout the period, the rivalry between the two superpowers was played out in multiple arenas: military coalitions; ideology, psychology, and espionage; sports; military, industrial, and technological developments, including the space race; costly defence spending; a massive conventional and nuclear arms race; and many proxy wars.

There was never a direct military engagement between the US and the Soviet Union, but there was half a century of military buildup as well as political battles for support around the world, including significant involvement of allied and satellite nations in proxy wars. Although the US and the Soviet Union had been allied against Nazi Germany, the two sides differed on how to reconstruct the postwar world even before the end of World War II. Over the following decades, the Cold War spread outside Europe to every region of the world, as the US sought the "containment" of communism and forged numerous alliances to this end, particularly in Western Europe, the Middle East, and Southeast Asia. There were repeated crises that threatened to escalate into world wars but never did, notably the Berlin Blockade (1948-1949), the Korean War (1950-1953), the Vietnam War (1959-1975), the Cuban Missile Crisis (1962), and the Soviet-Afghan War (1979-1989). There were also periods when tension was reduced 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 following Soviet leader Mikhail Gorbachev's summit conferences with United States President Ronald Reagan, as well as Gorbachev's launching of reform programs: perestroika and glasnost.