Polish-Ukrainian relations were among the most complicated in the Second Polish Republic. They can be described as complex, difficult and at times historically painful. The national aspirations of the eastern neighbor were contrary to the assumptions of the policy of an independent Republic of Poland, which further complicated the situation. The negative attitude of narrow but ideologically significant group of Ukrainians, along with their terrorist, sabotage and propaganda activities directed against the Polish state, clearly threatened the sense of security of Poles. The article deals with the activities of the Ukrainian Military Organization (UWO) in Lesser Poland Voivodeship in the years 1921-1926 and its negative impact on the region and relations between the Polish and Ukrainian communities. The aim of the article is to examine the effects of UWO terrorist and sabotage actions in the years 1921-1926 in Lesser Poland Voivodeship. The following research problems were defined: what was the attitude of the nationalist organization UWO towards Poland and Poles? What methods and forms of activity did the UWO use in Lesser Poland Voivodeship in 1921-1926? What impact did the effects of this activity have on the Polish state and Poles? Based on the conducted scientific query and analysis of available literature, the following hypotheses were adopted: firstly, the nationalist organization UWO had a hostile attitude towards Poland and Poles from the very beginning of its activity; secondly, in the years 1921-1926 in Lesser Poland Voivodeship, the UWO carried out numerous acts of sabotage, sabotage and terrorist acts directed against the Polish authorities and Poles; thirdly, the terrorist and sabotage activities of the UWO resulted in a threat to the existence of the Polish state and a significant reduction in the level of security among the inhabitants of Lesser Poland Voivodeship. The document analysis method was used as part of the literature research on the subject. Discussing the activities of the UWO in the international context and Polish-Ukrainian relations, the article critically assesses the effects of radical nationalist activities on both nations. From the beginning of its existence, nationalists from the Ukrainian Military Organization directed their actions exclusively against Poland, demanding the right to represent all Ruthenians/Ukrainians within the borders of the Second Polish Republic. Unfortunately, the effects of this approach turned out to be disastrous both for them and for the Poles from Lesser Poland Voivodeship. Ultimately, the work highlights the essential need for historical memory and contextual understanding to avoid repeating the mistakes of the past and to build lasting, understanding relationships between nations in the future.