Abstract: The standoff in the Gulf that commenced in May 23 between Qatar and Saudi Arabia, Bahrain, and the United Arab Emirates (UAE) presents the greatest challenge to the Gulf Cooperation Council (GCC) since the Iraqi invasion of Kuwait in August 1990 and the Gulf War in January–February 1991. The demands made of Qatar by the trio of fellow GCC states have laid bare the tensions in the GCC that have for years complicated moves toward any meaningful form of collective defense cooperation. In addition, the fallout from the spat threatens to split the GCC along multiple lines and open inroads for new participants in regional security structures. The involvement of countries such as Turkey, and potentially Russia and Iran, is likely to widen existing fractures within the GCC and weaken the web of partnerships with Western states that have formed the cornerstone of the post-1991 Gulf security architecture. Two threads run through regional security structures in the Gulf and connect the past to the present. The first is the presence of external forces with their own interests in maritime and regime stability, while the second is the imbalance of power and difference in threat perception between the three larger states—Iran, Iraq, and Saudi Arabia—and the five smaller states—Bahrain, Kuwait, Oman, Qatar, and the UAE. While the exact nature of this imbalance has fluctuated considerably over time, ithas contributed to a marked preference for bilateralism over multilateralism in most matters of national security, and created trajectories that may widen further with the Qatar crisis.