Patterns of military violence against civilians vary considerably, between conflicts and within them. This article explores why some combatants are more likely than others to engage in violence against civilians, focusing on a particular subset of such violence, violence that is not planned or authorized by military superiors, termed opportunistic violence. In a sample of Israeli combat soldiers from the Second Intifada, opportunistic violence was found to be strongly associated with duration of deployment among civilians. Long deployments erode social and moral norms, raising the likelihood that combatants will act opportunistically. However, the relationship between deployment duration and opportunistic violence was moderated by unit structure, such that long deployments were more likely to cause opportunistic violence in units with weak command structures. These findings suggest that the deleterious effects of long deployments are not inevitable but can be limited to a large extent through well-functioning structures of command and discipline.