A variety of domestic characteristics of states affect their propensities to armed conflict, including power, regime type, wealth, and economic strength (in addition to the dyadic characteristics of power differential, alliances, proximity, and the peace-learning process). Compared to these, religion is an understudied characteristic. Religions instill norms and ethics for the use of force just as secular ideologies often do. These war ethics influence the propensities to armed conflict of the states whose people and leadership adhere to those religions. Whether religious war ethics raise or lower those propensities depends on how permissive or restrictive they are. I show the empirical effect of those religious war ethics, working through states’ populations, on states’ probabilities to initiate armed conflicts against other states. The Christian war ethic is more restrictive and Christian populations are negatively correlated with states’ propensities to resort to force. The Islamic war ethic is more permissive and Muslim populations are positively correlated. The effect of religion is often strong and statistically significant, even after introducing conventional controls