The U.S.-led effort to establish a missile defense architecture for the Persian Gulf has been slower and less successful than the United States had hoped, mainly due to an unwillingness and inability to cooperate among the Gulf Security Council nations whose nations the system is designed to defend. Given, inter alia, Iran’s growing ballistic missile arsenal and unease with the Joint Comprehensive Plan of Action in Gulf Arab capitals, security reassurances to the Gulf monarchies will become simultaneously more important and more difficult to make credible. In this environment, missile defense will be an important, but by no means sufficient, mechanism for assuring the Arab Gulf states. Cooperation on missile defense with the Gulf monarchies should continue, but with a realistic understanding of what is possible given the current chaos and political dynamics of the region.