Following the breakup of the Russian Empire in the aftermath of World War I for a brief period, from 1918 to 1920, Armenia was an independent republic. In late 1920, the communists came to power following an invasion of Armenia by the Red Army; in 1922, Armenia became part of the Transcaucasian SFSR of the Soviet Union, later on forming the Armenian Soviet Socialist Republic (1936 to 21 September 1991). In 1991, Armenia declared independence from the USSR and established the second Republic of Armenia.