Military alliances are a crucial and much-studied aspect of world politics. They are also a defining feature of US grand strategy. During its tenure as the leading state in the international system, the United States has assembled an unprecedented network of security relationships that extend across the globe, including its multilateral alliance with other members of the North Atlantic community, its bilateral alliances in East Asia and Oceania, and its informal but close partnerships with states throughout the Middle East. Indeed, these security relationships are regularly cited as a unique source of advantage in competitions with major power rivals, as well as a potential source of vulnerability for a hegemon hoping to avoid overextension. Understanding why alliances emerge, what form they take, and the roles they play not only is of interest to academics, therefore, but is also at the center of current foreign policy debates over deepening versus downsizing Washington’s commitments.