The Soviet Union had ceased to recognize the Polish state at the start of the invasion . As a result , the two governments never officially declared war on each other . The Soviets therefore did not classify Polish military prisoners as prisoners of war but as rebels against the new legal government of Western Ukraine and Western Byelorussia . The Soviets killed tens of thousands of Polish prisoners of war . Some , like General Józef Olszyna-Wilczyński , who was captured , interrogated and shot on 22 September , were executed during the campaign itself . On 24 September , the Soviets killed 42 staff and patients of a Polish military hospital in the village of Grabowiec , near Zamość . The Soviets also executed all the Polish officers they captured after the Battle of Szack , on 28 September . Over 20,000 Polish military personnel and civilians perished in the Katyn massacre
Katyn massacre
Katyn massacre
Occupation of Poland (1939–1945)