This study's objective is to explore the psychological defense mechanisms of Ukrainian service members as a regulatory system for personality stabilization that influences combat mission effectiveness. The study was carried out during 2019–2020. The respondents were 270 military personnel of the ground forces, who had gained experience in the Anti-Terrorist Operation hostilities in the East of Ukraine in 2017–2020. We used psychodiagnostic methods such as the Lifestyle Index by Plutchik, Kellerman, and Conte; Lazarus’s Coping Test; and Leontiev’s Meaningful Life Orientations (...) Test. Our work confirmed our research hypothesis that psychological defense mechanisms as part of the regulatory system of personality stabilization predetermine the effectiveness of the combat missions performed by military personnel of special branches. Military psychologists can use the results during their official training (psychological training and psycho-corrective measures) in order to acquire among the troops the required effectiveness in combat. Defense mechanisms will also contribute to the observance of military ethics, not least when faced with military immorality on the part of adversaries. (shrink)