How does conflict affect prosocial and parochial preferences within a society? Our research considers the case of recent violence in Donbas, Ukraine where ethnic Russian separatists are battling the Ukrainian military. To evaluate social preferences, we utilize a non-costly dictator game with ethnic treatments among young ethnic Ukrainian male combatants and noncombatants in the eastern city of Kharkiv, which borders the Donbas region. At the onset of violence, we find no differences in how these men treat ethnic Russians in their local community compared to their own in-group. However, after a year of intense fighting with separatists in the nearby Donbas region, we find evidence of the erosion of fairness preferences and increased bias against ethnic Russians, especially among noncombatant civilians, underscoring how parochial responses to violence may extend beyond direct combat exposure mechanisms. Our results point to the short-term destabilizing effects of conflict on prosocial preferences with potential long-term consequences for entrenching parochial divisions.