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. These lands now form the backbone of modern Western Ukraine and West Belarus.
These, added to other posterior and more minor in comparison territorial gains from Romania, resulted in the Ukrainian Soviet Socialist Republic gaining 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. Some other Polish territory also invaded by the Soviet Union was given to Soviet Belarus.
History
On September 17, 1939 the Red Army entered Polish territory, acting on the basis of a secret clause of the Molotov–Ribbentrop Pact between the Soviet Union and Nazi Germany. The Soviet Union would later deny the existence of this secret protocol, claiming that it was never allied with the German Reich, and acted independently to protect the Ukrainian and White Ruthenian (modern Belarusian) minorities in the disintegrating Polish state. Composed of mostly ethnic Ukrainian Soviet troops under the command of Marshal Semyon Timoshenko, the Soviet forces occupied the eastern areas of Poland within 12 days, capturing the regions of Galicia and Volhynia with little Polish opposition, and occupying the principal city, Lwów, by September 22.
According to Volodymyr Kubiyovych, Soviet troops were greeted with genuine joy by the Ukrainian villagers due to the Polish government's discrimination against the Ukrainian minority in previous years. Not all Ukrainians trusted the Soviet regime responsible for the Ukrainian Famine of 1932–1933. In practice, the poor generally welcomed the Soviets, while the elites tended to join the opposition, despite supporting the unification of Ukraine.
Immediately after entering Poland's territory, the Soviet army helped to set up "provisional administrations" in the cities and "peasant committees" in the villages in order to organize one-list elections to the new "People's Assembly of Western Ukraine". The elections were designed to give the annexation an appearance of validity, but were far from free or fair. The voters had a choice of only one candidate, often a local communist or someone sent to western Ukraine from Soviet Ukraine for each position of deputy; the communist party commissars then provided the assembly with resolutions that would push through nationalization of banks and heavy industry and transfers of land to peasant communities. Elections took place on October 22, 1939; the official numbers reported participation of 93 percent of the electorate, 91 percent of whom supported the appointed candidates. Based on these results, the People's Assembly of Western Ukraine, headed by Kyryl Studynsky (a prominent academic and figure in the Christian Social Movement), consisted of 1,484 deputies. They met in Lwów on October 26–28, where they were addressed by Nikita Khrushchev and other representatives of the Ukrainian Soviet Socialist Republic. The assembly voted unanimously to thank Stalin for liberation and sent a delegation headed by Studynsky to Moscow to ask for formal inclusion of the territories into the Ukrainian SSR. The Supreme Soviet voted to do so on November 1, 1939 and on November 15 a law was passed making the former eastern Polish territories a part of the Ukrainian SSR.