Classic work suggests that intergroup conflict increases intragroup cohesion and cooperation. But how do group members respond when their peers refuse to cooperate? Simmel ([1908] 1955) argued that groups in conflict quell dissent by sanctioning group members and supporting centralized leadership systems. This claim has important implications, but little direct support. This research investigates how intergroup conflict shapes individuals' tendencies to sacrifice for their groups, enforce norms by sanctioning their peers, and relinquish decision-making autonomy to a leader. I test the predictions with two small group experiments, which find that conflict increases enforcement of norms when outgroup participation in conflict is high and increases contribution to the group regardless of outgroup participation in conflict. Evidence on support for leaders is mixed and suggests that the performance of the group may affect support for leaders. The research has broader theoretical implications for the study of group processes, collective action, and institutions. (Contains 3 figures, 3 tables, and 9 footnotes.)