Following German aggression on Poland , and in accordance with the secret protocol of Molotov – Ribbentrop Pact , Soviet forces invaded eastern Poland on September 17 , 1939 . As bulk of the Polish Army was concentrated in the west , fighting Germans , the Soviets met with little resistance and their troops quickly moved westwards . Polish authorities originally intended to organize anti-German resistance in Stanisławów Voivodeship '' ( see : Romanian Bridgehead ) '' , with Polish Army units planned to stand ground until the spring of 1940 when French attack on Germany was expected . However , the Soviet invasion of Poland rendered these plans ineffectual . It is estimated that prior to Soviet counter-offensive in the later part of World War II , over 18,000 Polish civilians in Stanisławów Voivodeship fell victims to OUN-UPA massacres
massacres
Massacres of Poles in Volhynia and Eastern Galicia
Stanisławów Voivodeship