Sexual integration of combat forces presents underappreciated challenges. Sex differences in physical capacity remain important in modern warfare, and the sexes also differ in combat-relevant psychological traits, including risk taking and aggressiveness. Moreover, group dynamics have consequences for unit cohesion and combat performance. Men more easily participate in coalitions organized to mete out violence, a tendency enhanced in the presence of intergroup competition. Men's coalitions require lower levels of investment and can persist for longer in the face of within-group conflict than women's coalitions. Combat units rely on cohesion to enable performance, and introduction of women tends to reduce cohesion because, among other reasons, men often find it difficult to trust women. The attributes that soldiers value in comrades are ones that would have been important for primitive warriors, including strength, physical courage, and other aspects of masculinity, which may mean that women cannot evoke trust in their male comrades the way other men can.