Soviet occupation of Western Ukraine

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.

In June 1940, the Soviet Union issued an ultimatum to the Kingdom of Romania, demanding the ceding of Northern Bukovina, a region with a large ethnic Ukrainian population. The Soviet troops rolled into Estonia as well in mid June 1940 and established the Estonian Soviet Socialist Republic of the Soviet Union there. The annexation of the Polish territories, which were added to Soviet Byelorussia, and the Ukrainian Soviet Socialist Republic alone, resulted in the Soviet state gaining 131,000 square kilometres (50,600 sq mi), and increasing its population by over seven million people.

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