Why do some external actors in the international system accept considerable risk in both blood and treasure to protect civilians from mass atrocities while others do not? What explains this variation in response? Mass atrocities – including war crimes, crimes against humanity, ethnic cleansing, and genocide - are complex social phenomena exhibiting stark temporal and spatial variation, spanning millennia and civilizations. Despite significant gains during the last three decades in scholarly understanding of atrocities, including causal mechanisms, technical advancements in early warning, and normative development related to State Responsibility to Protect civilians, the early 21st Century continues a grim record of state violence targeting civilians. This triptych deconstructs the theoretical prevention space in International Relations (IR) by presenting three interlinked papers that engage these central concepts and questions animating the atrocity prevention puzzle, eventually isolating one variable, political will, as both a crucial construct, and core explanatory variable, for theorizing and understanding external actor behavior in mass atrocity prevention and response. I make a knowledge claim that connects the three papers and serves as the central theme throughout. Paper One and Two provide descriptive and conceptual contributions to the literature on early warning and political will in atrocity scholarship. In Paper Three I deploy a Controlled Comparison of external decision making in response to atrocity violence in Libya and Syria using Mill’s Method of Concomitant Variation to argue that variation in political will (IV) explains much of the variation in state behavior (DV) in response to mass atrocities.