The Responsibility to Protect (R2P) invokes one of the most powerful moral and legal terms in contemporary international politics – namely, responsibility. The nature of the relationship between R2P and international law and morality, however, remains contested, giving rise to questions lying at the core of R2P's normative foundations. What is the source of R2P? To whom is this responsibility attributable, and under what circumstances? Does R2P give rise to legal obligations? Such questions challenge International Relations (IR) theorists to look beyond their discipline for more insightful tools and methods of analysis. In this article, we apply a broadened theoretical framework to explain the ongoing controversy about R2P. In Part II, we borrow tools from moral philosophy to identify the source and the bearer of the responsibility to protect in today's international society. In Part III, we draw on international legal scholarship to analyse whether R2P has emerged as a 'new' norm of customary international law. We find that international endorsement of R2P has helped to clarify existing obligations in international law, but that intrinsic ambiguities in its articulation currently limit R2P's capacity to entrench new obligations for states to protect strangers. At the same time, our finding that R2P is an example of 'soft law' leads us to conclude that R2P can nonetheless exert significant influence on how states interpret their legal obligations and, in the coming decade, it may also help catalyse diplomatic efforts to reform the international architecture for preventing and responding to mass atrocities.