Before the war , Poland had a substantial population of Ukrainian and Belarusian minorities living in her eastern , '' Kresy '' regions . After the Soviet invasion of eastern Poland on 17 September 1939 , those territories were annexed by the USSR . Following the German invasion of the Soviet Union in June 1941 , German authorities recruited Ukrainians and Belarusians who had been citizens of Poland before September 1939 for service in the Waffen-SS and auxiliary-police units , serving as guards in the German-run extermination camps set up by the Nazis in occupied-Poland , and to assist with anti-partisan operations . In District Galicia , the SS Galicia division and Ukrainian Auxiliary Police , made up of ethnic-Ukrainian volunteers , took part in widespread massacres and persecution of Poles and Jews . Also , as early as the September Campaign , the Organization of Ukrainian Nationalists ( '' Orhanizatsiya Ukrayins ' kykh Natsionalistiv '' , or '' OUN '' ) had been “ a faithful German auxiliary " carrying out acts of sabotage against Polish targets on behest of the Abwehr
massacres and persecution of Poles
Massacres of Poles in Volhynia and Eastern Galicia
Collaboration in German-occupied Poland