This article seeks to clarify the current legal status of a particular aspect of the 'responsibility to protect' principle-the idea that bystander states have a collective responsibility to protect populations beyond borders from mass atrocities when host states fail to do so. It outlines the development of this idea and argues that, while the legal force of key international statements on the 'responsibility to protect' principle may be weak at best, the International Court of Justice and the International Law Commission have offered bold declarations in recent years which do point towards the gradual development of legal duties for the extraterritorial protection of populations.