Ukrainian Soviet Socialist Republic
Eventually, after the creation of the Communist Party (Bolshevik) of Ukraine in Moscow, a third Ukrainian Soviet government was formed on 21 December 1919 that initiated new hostilities against Ukrainian nationalists as they lost their military support from the defeated Central Powers. Eventually, the Red Army ended up controlling much of the Ukrainian territory after the Polish-Soviet Peace of Riga. On 30 December 1922, along with the Russian, Byelorussian, and Transcaucasian republics, the Ukrainian SSR was one of the founding members of the Union of Soviet Socialist Republics (USSR).
Soviet annexation of Eastern Galicia, Volhynia and Northern Bukovina
The Soviet annexation of some 51.6% of the territory of the Second Polish Republic, where about 13,200,000 people lived in 1939 including Poles and Jews, was an important event in the history of contemporary Ukraine and Belarus, because it brought within Ukrainian and Belarusian SSR new territories inhabited in part by ethnic Ukrainian and Belarusian people, and thus unified previously separated branches of these nations. The postwar population transfers imposed by Joseph Stalin, and the Holocaust, solidified the mono-ethnic character of these lands by nearly a complete eradication of the Polish and Jewish presence there. Ukraine and Belarus achieved independence in 1991 after the fall of the Soviet Union and became nation states delineated by borders of the 50-year-old republics. "The process of amalgamation – wrote Orest Subtelny, a Canadian historian of Ukrainian descent – was not only a major aspect of the post-war period, but an event of epochal significance in the history of Ukraine."