The Union of Soviet Socialized Republics was a country that was founded by Vladimir Lenin in 1917 and lasted until 1991, when it broke up rapidly. It replaced Russia as the most powerful country in the region. The USSR was a single party state in which the Communist Party dominated. It was formed by the Bolsheviks after the overthrow and the eventual assassination of Tsar Nicholas II. Although it was an ally of the United Kingdom and the United States during World War II, the Soviet Union developed a strong rivalry with both countries for several decades following. This stand off became known as the Cold War. The Soviet Union began to decline in the 1980s, and Premier Mikhail Gorbachev enacted several reform acts. Despite this the nation collapsed in 1991. It was replaced by the following countries: Russia, Moldova, the Ukraine, Estonia, Latvia, Lithuania, Tajikistan, Uzbekistan, Turkmenistan, Armenia, Azerbaijan, Kazakhstan, and Georgia.