The peritraumatic cognitive process of mental defeat, the complete loss of inner resistance, has been identified as a key predictor of PTSD. Yet, most evidence on cognitive risk factors stems from industrialized countries where survivors typically report few traumata. Research from postconflict settings indicates that individual differences decrease with accumulating traumatic experiences, as almost everybody develops PTSD at extreme levels of trauma load. Would this leave less room for the impact of cognitive processes? In a sample of 227 Ugandan rebel war survivors, we investigated whether mental defeat influences trauma-related psychopathology in regression models accounting for cumulative trauma exposure. We found strong main effects of mental defeat on lifetime PTSD risk, current PTSD severity and dissociative symptoms, but no mental defeat × trauma load interaction effects. Our results indicate that peritraumatic mental defeat is central to understand individual differences in psychological reactions after single traumatic events as well as multiple traumatization.