Debates over how best to ensure appropriate conduct in battle typically draw a binary distinction between rule compliance and rule violation. This framing is problematic, excluding a critical third element of battlefield conduct, supererogation—that is, positive acts that go beyond what is demanded by the explicit rules of war. This article investigates this moral category of action; specifically, situations in which combatants refrain from taking the life of an enemy despite their moral and legal license to do so. It first (...) considers the moral tension between the duty of combatants to kill and battlefield mercy, and goes on to explore the factors that motivate the latter. The article then shifts to consider the significance of supererogation to the ongoing efforts to moderate the conduct of contemporary war. As the article illustrates, supererogatory restraint is constituted by values that when cultivated also incentivize adherence to the more explicit rules and standards of the battlefield. This is demonstrated through analysis of the conduct of Western special forces. The concept of supererogation is of further use when evaluating the origins and implications of “moral injury.” This is verified empirically in the context of armed unmanned aerial vehicles. (shrink)