Around 150,000 Polish civilians were killed by Soviets between 1939 and 1941 during the Soviet Union 's occupation of eastern Poland ( Kresy ) , and another estimated 100,000 Poles were killed by the Ukrainian Insurgent Army ( UPA ) in the regions of Wołyń and Eastern Galicia between 1943 and 1944 in what became known as the Wołyń Massacres . The massacres were part of a vicious ethnic clensing campaign waged by Ukrainian nationalists against the local Polish population in the German-occupied territories of eastern Poland
Wołyń Massacres
Massacres of Poles in Volhynia and Eastern Galicia
Poland