This article tracks the appearance and evolution of the concept of civilian assumption of risk in the successive editions of the United States Department of Defense’s Law of War Manual. Recognizing that civilians present in or on military objectives could either be wholly or partially discounted in assessing the legality of a strike threatens core protections guaranteed by the principle of proportionality in international humanitarian law. That recognition also leaves the door open to the emergence of a third category between combatants and civilians. The article retraces the origins of the concept of civilian assumption of risk and considers, in particular, the situation of civilians working in or on military objectives. It argues that the notions of civilian assumption of risk and selective application of proportionality laid down in the Law of War Manual are not supported by law or practice and that any attempt at introducing an intermediary category of ‘quasi-civilians’ or ‘quasi-combatants’ should be rejected. References to civilian assumption of risk only comply with the law if they are understood to mean that civilians working in or on a military objective expose themselves to the risk of an attack complying with proportionality. Because it is a legally flawed concept fraught with practical difficulties in its implementation, the concept of civilian assumption of risk could have serious repercussions on how military officers understand their obligations and the protections afforded by international humanitarian law.