The aversion of the American and other Western societies to fatalities among their own armed forces and the depressing effect of military casualties on war support are well documented. We know much less about U.S. respondents’ concern for non-compatriot, civilian casualties. U.S. military doctrine relies heavily on international law to meet expectations of appropriate battlefield conduct, but whether attitudes towards wartime killings track legal principles is likewise unclear. This study draws on an original sur- vey experiment with 3000 U.S. citizens to elucidate popular attitudes towards killing in war. We test the extent to which instrumental considerations, moral precepts, and legal principles respectively account for respondents’ attitudes towards individual at- tacks. Moreover, we shed light on the role of concern for civilian casualties in explaining respondents’ support for a war when this concern must be weighed against considerations of aggregate costs, military casualties, and the likelihood of a war’s success. Our experimental design allows us to estimate the individual and interactive effects of these variables on attitudes towards wartime killing. This affords a more nuanced empirical grasp of the potentially restraining power of public opinion on the use of military force and allows us to test our conceptualisation of international law as stable between prior normative beliefs and instrumental considerations.