What makes some states more militarily powerful than others? A growing body of research suggests that certain ‘non-material’ factors significantly affect a country’s ability to translate resources into fighting power. In particular, recent studies claim that democracy, Western culture, high levels of human capital, and amicable civil-military relations enhance military effectiveness. If these studies are correct, then military power is not solely or even primarily determined by material resources, and a large chunk of international relations scholarship has been based on a flawed metric. The major finding of this article, however, suggests that this is not the case. In hundreds of battles between 1898 and 1987, the more economically developed side consistently outfought the poorer side on a soldier-for-soldier basis. This is not surprising. What is surprising is that many of the non-material factors posited to affect military capability seem to be irrelevant: when economic development is taken into account, culture and human capital become insignificant and democracy actually seems to degrade warfighting capability. In short, the conventional military dominance of Western democracies stems from superior economic development, not societal pathologies or political institutions. Therefore, a conception of military power that takes into account both the quantity of a state’s resources and its level of economic development provides a sound basis for defense planning and international relations scholarship.