What Does the ‘Responsibility to Protect’ Require of States in Ukraine?

This article considers what the responsibility to protect populations from atrocity crimes (R2P) requires States to do in Ukraine. It begins by considering how the R2P’s ‘three-pillar’ implementation strategy is to be understood in relation to a situation in which atrocities are being perpetrated not by a State against its own population but by an aggressor State; and then considers the parallels between what is required of States by the political commitment to the R2P, and what is required by the legal obligations to prevent genocide and to cooperate to end serious breaches of peremptory norms. The second part of the article then considers a series of options that may be pursued by States in fulfilment of their R2P and corresponding legal obligations, from options falling short of military intervention (including legal proceedings, economic and diplomatic sanctions and military assistance) through to the possibilities of unilateral military intervention and UN peacekeeping.