The consequences of implementing the assumptions of the ideology of Ukrainian nationalism were especially tragic for the Polish population living in Volhynia and Eastern Lesser Poland. The number of Poles murdered by the OUN-UPA and other armed Ukrainian nationalist formations in 1939-1948 is not known exactly, but it is estimated that it was from 80,000 to 150,000 people. This article is an attempt to answer the question of what the determinants of Ukrainian nationalism were. For this purpose, it refers to the historical, civilization and cultural background on which the Ukrainian soul have been shaped for centuries. This foundation prepared the space for the emergence of a criminal ideology, and then for its implementation.