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.