What role do moral norms play in citizen attitudes towards governments use of military force? Most work on the morality-war link is either non-experimental or squarely focused on moral values rather than norms. We address this gap by developing a concept of moral norms concerning interstate war. Following behavioral psychology work on norms, we conceptualize moral norms about war as second-order expectations about when other members of society find war normatively appropriate. We present a survey-experimental framework to measure moral norms about war and gauge the effect of moral norms on support for war in different contexts.