Answer:
The USSR came into being after the Russian Revolution and Civil War. In 1922 the Treaty of the Soviet Union was signed between Russia, (newly named the Russian Soviet Federated Socialist Republic), Ukraine, Belorussia and the Transcaucasus Federation (Georgia, Armenia and Azerbaijan). Other countries either joined the USSR voluntarily or were dragged along by their earlier associations with Imperialist Russia. The USSR ultimately consisted of the following 15 SSR's -
Armenia
Azerbaijan
Belorus
Estonia
Georgia (Gruziya)
Kazakhstan
Kirgizstan
Latvia
Lithuania (Litva)
Moldova
Russia (Russian Soviet Federated Socialist Republic)
Tadjikistan
Turmenistan
Ukraine
Uzbekistan
Since the break up of the USSR, all of these SSR's have returned to independent states although some still rely to varying degrees on Russia for some government functions. Also, some of them have resumed the various ethnic hostilities that the USSR had kept in check for many years.