Military Alliances and Public Support for War

Abstract How do military alliances affect public support for war to defend victims of aggression? We offer the first experimental evidence on this fundamental question. Our experiments revealed that alliance commitments greatly increased the American public's willingness to intervene abroad. Alliances shaped public opinion by increasing public fears about the reputational costs of nonintervention and by heightening the perceived moral obligation to intervene out of concerns for fairness and loyalty. Finally, although alliances swayed public opinion across a wide range of circumstances, they made the biggest difference when the costs of intervention were high, the stakes of intervention were low, and the country needing aid was not a democracy. Thus, alliances can create pressure for war even when honoring the commitment would be extremely inconvenient, which could help explain why democratic allies tend to be so reliable. These findings shed new light on the consequences of alliances and other international legal commitments, the role of morality in foreign policy, and ongoing debates about domestic audience costs.