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.