Abstract: This paper examines how military alliances affect public support for war. Our survey-based experiments show that alliance commitments powerfully influence mass preferences about whether to intervene abroad. Both written and unwritten alliances had massive effects on support for sending U.S. forces. Two mechanisms drove these effects: concerns about reputation in international affairs, and concerns about the morality of leaving an ally hanging. The effects of alliances proved robust across a wide range of contextual factors, including the stakes for the United States, the anticipated costs of intervention, the political regime of the victim, and the region in which the conflict was located. Overall, our experiments imply that alliances bind across a wide range of conditions, primarily because alliance commitments increase both the reputational and moral costs of nonintervention.