The Republics of the Union of Soviet Socialist Republics or the Union Republics () were ethnically based administrative units of the Union of Soviet Socialist Republics (USSR). The Soviet Union was created by the treaty between the soviet socialist republics of Belarus, Ukraine, Russia, and the Transcaucasian Federation, by which they became its constituent republics. For most of its history, the USSR was a highly centralized state despite its nominal structure as a federation of republics; the decentralization reforms during the era of "Perestroika" ("Restructuring") and "Glasnost" ("Openness") conducted by Mikhail Gorbachev are cited as one of the factors which led to the dissolution of the USSR in 1991.