Soviet annexation of Eastern Galicia and Volhynia

On the basis of a secret clause of the Molotov–Ribbentrop Pact, the Soviet Union invaded Poland on September 17, 1939, capturing the eastern provinces of the Second Polish Republic. Lwów (present-day Lviv), the capital of the Lwów Voivodeship and the principal city and cultural center of the region of Galicia, was captured and occupied by September 22, 1939 along with other provincial capitals including Tarnopol, Brześć, Stanisławów, Łuck, and Wilno to the north. The eastern provinces of interwar Poland were inhabited by an ethnically mixed population, with ethnic Poles as well as Polish Jews dominant in the cities, and ethnic Ukrainians dominating the countryside and overall. These lands now form the backbone of modern Western Ukraine and West Belarus.^[1][2]

These, added to other posterior territorial gains from Romania, resulted in the Ukrainian Soviet Socialist Republic gaining 131,000 square kilometres (50,600 sq mi) in area, and increasing its population by over seven million people from 1938 to 1941. Eastern Galicia and Volhynia were the regions that contributed the most to this.^[3][4] Some other Polish territory also invaded by the Soviet Union was given to Soviet Belarus.