How can we explain variations in the approaches of regional organizations (ROs) to cross-border security threats, especially those derived from internal conflicts such as civil wars and unconstitutional changes of government? On the one hand, regional security threats appear regularly throughout the world. Pulling in the opposite direction, governments steadfastly insist on their sovereign right to conduct business within their borders as they see fit. In practice, some ROs have intervened more than others in the internal conflicts of their member states. How can we explain the difference? This article argues that ROs are intrusive when their member states share a belief that a state is too fragile to survive without intervention. Such a shared belief about state strength is fostered by historical developments, notably national economic development that strengthens state resources available to a regime, and past willingness of a regime to accept intervention. The argument is developed through a comparative analysis of the Association of Southeast Asian Nations and the Economic Community of West African States, respectively the world’s least and most intrusive ROs.