a former federation of communist republics that occupied the northern half of Asia and part of eastern Europe; capital, Moscow. Created from the Russian empire in the aftermath of the 1917 Russian Revolution, the Soviet Union was the largest country in the world. After World War II, it emerged as a superpower that rivaled the US and led to the Cold War. After decades of repression and economic failure, the Soviet Union was formally dissolved in 1991. Some of its constituents joined a looser confederation, the Commonwealth of Independent States. Full name Union of Soviet Socialist Republics