Union of Soviet Socialist Republics (often shortened to USSR or Soviet Union), a nation of Eurasia that existed from 1922 until 1991. It was the world's most powerful Communist country. The Soviet Union was formed out of most of the territory of czarist Russia, an empire that fell in the Russian Revolution of 1917. The USSR was made up of 15 republics—called soviet socialist republics, or union republics—under the control of a central government. In 1991 the Soviet Union collapsed and broke up into 15 independent countries.
The USSR became what is often called a "superpower." It served as a political and economic model for all of the other Communist countries. (Some of these, however, eventually broke away from the Soviet model.) During the period known as the Cold War, the USSR was the greatest rival of the United States in international affairs. Both countries established economic and military ties with many countries, and both vied for prestige among the world's neutral nations. Also during this period, the Soviet Union, by virtue of its military strength, dominated the governments of most of the nations of eastern Europe.