Soviet annexation of Western Belorussia
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. 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.
Source: Wikipedia — Soviet annexation of Western Belorussia (CC BY-SA 4.0)