This article uses an original data set, the Wartime Civil-military Relations Data Set, to test arguments about the causes of victory and defeat in war. Our analysis provides strong initial support for the notion that civil-military relations powerfully shape state prospects for victory and defeat. Specifically, states whose militaries have a significant internal role or whose regimes engage in coup-proofing appear to have a substantially lower probability of winning interstate wars, even when we account for the role of other important variables, including regime type and material capabilities. Crucially, our measures of civil-military relations include coup incidence but also move beyond it to detect more subtle indicators of civil-military relations. The resulting analysis should give us confidence in acknowledging the importance of nonmaterial variables in explaining war outcomes, while also paving the way for further research that can utilize and extend the data set.