This chapter analyses R2P’s third and most controversial pillar, which calls upon the international community to react to the commission of mass atrocity crimes when host States fail in their protection responsibilities. As such, this chapter investigates the legal underpinnings of and States’ attitudes towards this pillar to situate it within existing obligations and norms, including through the consideration of relevant provisions of the Genocide Convention, the four Geneva Conventions, and the Articles on State Responsibility. This analysis is complemented with a discussion of how R2P’s Pillar 3 must be strengthened or developed with view to achieving a robust R2P framework that is not employed on a case-by-case basis, but rather serves as an effective and legally grounded tool to bring an end to all mass atrocity crimes. Recognising that the full enforceability of Pillar 3 is contingent in many respects upon overcoming selectivity as well as abuse of the veto power inherent within the UN Security Council, this chapter devotes special consideration to identifying means through which the veto can be curtailed or even circumvented in mass atrocity situations.