Although humans qualify as one of the most cooperative animal species, the scale of violent intergroup conflict among them is unparalleled. Explanations of the underlying motivations to participate in an intergroup conflict, however, remain unsatisfactory. While previous research shows that intergroup conflict increases individually costly behavior to the benefit of the in-group, it has failed to identify robust triggers of aggressive behavior directed at out-groups. Here, we present a controlled laboratory experiment which demonstrates that such aggression can be provoked systematically by manipulating the extent to which the own group is perceived to be on the offensive or the defensive side of a conflict. We find direct and causal evidence that the motivation to protect the in-group not only is a predictor of retaliatory aggression, but also promotes preemptive offensive actions against out-groups if they pose a potential threat. This finding improves our understanding of the escalation of intergroup conflicts and may have important implications for their prevention, as we find in our experiment that removing out-group threat substantially reduced intergroup aggression and led to full peace.