This paper examines why members of a security alliance raise disputes with their allies in public and the impact of such disputes on alliance relations. Allies sometimes have problems when they cooperate under a security alliance. Members of an alliance may have different views about how to enforce the alliance agreement; they may find their allies incapable of providing the security benefit they promised; or they may claim that their allies’ actions are harmful to mutual security interests. Allies may raise disputes to address these concerns on alliance cooperation.