Why are some militaries so much more effective on the battlefield than others? Specifically, why are militaries more or less cohesive? Drawing on recent work on military inequality theory, I theorize that political communities with salient socioeconomic divisions will have poor military cohesion. I argue that when these divisions are mobilized and incorporated into the military, it will result in inter-group distrust and a lack of sense of shared fate between elites and out-group members. These two mechanisms will lead to worse military performance. I also conduct an initial empirical test of the relationship between socioeconomic inequality and battlefield performance for 627 belligerents in conventional wars from 1800 to 2011 using data from Project Mars and V-Dem. The results of this analysis show limited evidence for the existence of a negative association between inequality and military effectiveness, but this initial analysis has significant limitations. These findings highlight the need for further theoretical and empirical work on the link between socioeconomic inequality and military performance.