This article explores the interface between religion and international humanitarian law (IHL), and the degree to which they might complement and reinforce each other. It examines some of the challenges inherent in regulating armed conflict and the understandable limitations of IHL in this respect, and argues that re-engagement with IHL's religious roots can help to alleviate them. Engagement with religious circles mobilizes the vast resources of religions to increase knowledge of IHL and corresponding religious norms, thereby enhancing their legitimacy across religious and cultural divides. This is most effective when comparative study of IHL and religious teachings stimulates mutual learning and debate, in which both correspondences and differences are embraced. In the absence of a strong legal enforcement regime, religions can reinforce military ethics by tapping deeply into the identities, motivations and moral values of many belligerents, and possess powerful means to socialize the rules of war and improve voluntary compliance. Introspective religious practices encourage the moral self-reflection that is most effective at internalizing norms in this respect, as well as providing belligerents with the spiritual and psychological support needed to bolster their resilience and enable them to perform with precision and restraint.