The goal of this study is twofold. First, it seeks to move beyond the exploration of motivations for understanding why the United States launches some humanitarian interventions and avoids others. Second, it initiates a theory building process to map the complex international and domestic environment that frames American humanitarianism. To explain the selectivity of U.S. engagement, the article establishes a typology of actors, restraints, and concerns involved in the humanitarian policy-making process. It then presents a theory of coevolution that serves as a framework for understanding the interactive and diffusive dynamics between policy makers and their broader operating environment. With illustrative case studies on Operation Provide Comfort in Iraq (1991), Operation Allied Force in Kosovo (1999), and Operation Unified Assistance in response to the Asian Tsunami (2004), this study suggests that U.S.-led humanitarian interventions are part of larger episodes of engagement that hold consequences for subsequent involvements. It finds that altruistic interventions are often blurred with self-interested power pursuits, as American humanitarianism is the product of a confluence of domestic political factors, historical milieu, and international normative advancement.