context: The Cold War drew to a close in the late 1980s and the early 1990s. The United States under President Ronald Reagan 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 dominant military power, though Russia retained much of the massive Soviet nuclear arsenal.
question: సోవియట్ యూనియన్ ఏ సంవత్సరంలో కూలిపోయింది?
answers: 1991