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. These lands now form the backbone of modern Western Belarus.

The annexation of the territories increased the area of the Belarusian SSR by 108,000 square kilometres (42,000 sq mi) and almost doubled its population to 10.4 million people, of whom 4.8 million were newly incorporated.