The norm of civilian immunity, which holds that civilians must not be intentionally targeted in war or subjected to mass killing, is widely supported and considered a jus cogens principle of international law. Yet not only does mass killing remain a recurrent feature of world politics, but perpetrators sometimes avoid criticism or punishment. This article argues that the paradox can be explained by understanding that civilian immunity confronts a protracted struggle with competing ideologies, some of which have proven resilient, and that decisions about how to interpret the norm in specific cases are subject to intervening contextual variables.