Can armed groups socialize combatants to norms of restraint – in essence, train soldiers to adopt norms of international humanitarian law on the battlefield? How can social scientists accurately measure such socialization? Despite being the central focus of organizational and ideational theories of conflict, studies to date have not engaged in systematic, survey-based examination of this central socialization mechanism theorized to influence military conduct. This study advances scholarly understanding by providing the first comparative, survey-based examination of combatant socialization to norms of restraint, using surveys and interviews with US Army cadets at the US Military Academy (USMA), Army Reserve Officer Training Corps (ROTC), and active duty Army combatants. Additionally, to better understand ‘restraint’ from combatants’ perspective, this study introduces the concept of the ‘combatant’s trilemma’ under which combatants conceptualize civilian protection as part of a costly trade-off with the values of military advantage and force protection. Survey results hold both positive and negative implications for socialization to law of war norms: military socialization can shift combatants’ preferences for battlefield conduct. However, intensive norm socialization may be required to shift combatants’ preferences from force protection to civilian protection norms. Study findings hold significant implications for understanding violence against civilians in conflict and for policies to disseminate civilian protection norms in armed groups worldwide.