To pre-empt future Polish efforts at re-establishing Poland 's pre-war borders , in 1943-1944 some UPA military units carried out large-scale ethnic cleansing against Polish people . Historians estimate that 100,000 Polish civilians were massacred in Volhynia and Eastern Galicia
against Polish people
Massacres of Poles in Volhynia and Eastern Galicia
Organization of Ukrainian Nationalists