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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, and in 1922, Armenia became part of the Transcaucasian SFSR of the Soviet Union, later forming the Armenian Soviet Socialist Republic (1936 to September 21, 1991). In 1991, Armenia declared independence from the USSR and established the second Republic of Armenia.

What part of the USSR did Armenia join?
the Transcaucasian SFSR