An author focuses on the reasons of the creation by the Regional Unit of the Organization of Ukrainian Nationalists (Bandera followers) of the Ukrainian Insurgent Army in Volhynia and Polesie and the anti-Polish ethnic cleansing operation carried out in these territories. The article draws the conclusion that these reasons were determined, first of all, by the war factors, which, in turn, defined the time of carrying out of this operation as well as that of the Armia Krajowa [Home Army] activists’ reaction actions. Describing the milestones of the confrontation of the Ukrainian and Polish clandestine organizations in the territory of pre-war Polish Volhynian Voivodeship in 1943–1944, the author specifies the principle difference, which, according to the author, existed in the nature and ultimate goal of the mutual armed actions carried out by the detachments of the UPA and the AK. The article describes the polemics with some points of view on this problem, widespread in the newest Polish and Ukrainian historiography.