Cold War

The United States, the United Kingdom and the Soviet Union cooperated against the Axis powers during the Second World War, but their alliance rested on differing political systems and competing expectations for postwar Europe. After Germany's defeat in 1945, disagreements over security, political influence and the future of Eastern Europe intensified. The Soviet Union established governments aligned with its interests in much of Eastern Europe, while the United States backed Western European recovery and a policy of containing the spread of communism. Historians commonly trace the Cold War's origins to this postwar breakdown; rather than a single declaration or battle, it became a hardened rivalry by 1947–48.

The United States and the Soviet Union organized competing alliances and sought influence through diplomacy, economic assistance, military power and public messaging. Nuclear weapons made direct war between them extraordinarily dangerous, so their competition was expressed in an arms buildup and recurring crises as well as in regional conflicts where they supported opposing sides. These conflicts had their own local causes and participants; the countries and communities involved were not simply extensions of either superpower. The nuclear standoff also made negotiation and arms-control agreements important parts of the relationship.

The rivalry was not a constant escalation. Periods of heightened tension alternated with diplomacy and efforts to limit risks. The 1962 Cuban missile crisis brought the United States and Soviet Union close to a nuclear confrontation, after which the two governments negotiated the withdrawal of the missiles at its center. In later decades, arms-control talks attempted to constrain parts of the nuclear competition. Meanwhile, the relationship among communist states changed too: the split between China and the Soviet Union showed that the Cold War was not a simple two-sided division of the world.

During the 1980s, reforms under Soviet leader Mikhail Gorbachev and mounting demands for political change weakened communist rule in Eastern Europe. The Berlin Wall opened in November 1989, and communist governments across the region fell or changed over the following years. These transformations did not end every international tension, but they marked the collapse of the European order that had defined much of the Cold War. In December 1991 the Soviet Union formally dissolved into independent states; 1991 is therefore commonly used as the endpoint of the era.