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. This paper examines the 187 bilateral alliance treaties between 1945 and 2007 with original data on alliance disputes. The large-N analysis shows that the rise of external threats, power preponderance between allies, and joining a democracy have a positive impact on members’ decisions to raise public disputes. This paper further finds that public disputes increase the risk of treaty violations. Alliances that have experienced open arguments about security cooperation are less likely to be sustained.