Recent work in the alliance politics literature has highlighted various “strategies of reassurance,” including public reaffirmations of security guarantees, personal visits, economic sanctions, and force deployments. We argue that this extensive literature has overlooked a critical element in American reassurance of its allies: perceptions of an American presidency ready, willing, and able to act unilaterally (i.e., without authorization from Congress). Specifically, while allies seek for an American commitment to be “automatic” in case of attack, each U.S. defense pact contains a procedural clause conditioning the American commitment on its “constitutional processes.” This creates a “commitment gap” between the level of commitment required to effectively reassure allies and the level actually provided by lawmakers in the treaty text. In a self-help world, allies are highly sensitive to this disparity, pressuring the American executive branch to “bridge the gap” through means such as broad assertions of presidential power, demonstrative unilateral uses of force, and tripwire deployments, all of which constitutionally facilitate action absent congressional approval. The article illustrates the logic of the theory through case studies of American alliances with NATO, South Korea, the Philippines, and Thailand. It concludes by considering implications for the efficacy of tripwire deployments, contemporary efforts to reform the war powers, and broader debates over American grand strategy. The argument suggests that grand strategies that emphasize the importance of alliances effectively require an image of an imperial presidency.