Excerpts from Books and Wikipedia
"Essentially, the union was a re-creation of the old tsarist Russian Empire shorn of Poland and Finland, which had wrested their independence in a civil war following the 1917 Bolshevik Revolution. Always dominated by Russia, the USSR was initially composed of just four republics--Russia, Ukraine, Belorussia, and Transcaucasia--but as the Bolsheviks consolidated there control, more republics joined: Armenia, Azerbaijan, Georgia, Kazakhstan, Moldavia, Uzbekistan, Tajikistan, and Turkmenistan. In World War II, the Baltic republics of Estonia, Latvia, and Lithuania were forcibly annexed, too.… It extended communist control over Eastern Europe and communist influence across the entire world." [1001 Days]