Security alliances lie at the heart of US global strategy. They provide Washington with the capacity to project power internationally while at the same time exerting influence in regions where the United States has endur-ing political and economic interests. No region is more important to the US than the Asia-Pacific, and along with the Bretton Woods system, the so-called ‘hub and spokes’ system of bilateral alliances has been integral to American global strategy over the past seventy years. The US–Republic of Korea (ROK) alliance has been a central feature of the American presence in Asia since the 1950s. Along with the US–Japan alliance, the US–ROK alliance has been essential to the capacity of successive US administrations to project political influence and military power throughout Asia. In this chapter, I address two main questions. First, how has the US–ROK alliance evolved since the 1953 Mutual Defense Treaty? And second, how is the US–ROK alliance likely to develop over the next decade? I argue that while the alliance has evolved through various phases and has acquired greater institutional depth over time, several consistent themes have been apparent. The most prominent is the tension between entrapment and abandonment fears. These have been particularly salient during periods where the alliance has encountered turbulence – most notably during the 1970s – but it is notable that entrapment/abandonment anxieties have also been on display when the alliance has been relatively stable. Another consistent theme of the US–ROK alliance has been a persistent misalignment between intimate military-operational ties and patchy political-strategic relations. Although the gap has narrowed, even today the institutional depth of the military-operational relationship between the US and the ROK is stronger than the variable political-strategic coordination between Washington and Seoul.