The Soviet Union had ceased to recognise the Polish state at the start of the invasion . Neither side issued a formal declaration of war ; this decision had significant consequences , and Smigly-Rydz would be criticised for it . The Soviets killed tens of thousands of Polish prisoners of war , some 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 1939 . 22,000 Polish military personnel and civilians perished in the Katyn massacre . Torture was used by the NKVD on a wide scale in various prisons , especially those in small towns
Katyn massacre
Katyn massacre
Soviet invasion of Poland