The toll of civil conflict is largely borne by civilian populations, as warring factions target non-combatants through campaigns of violence. But significant variation exists in the extent to which warring groups abuse the civilian population: across conflicts, across groups, and within countries geographically and over time. Using a new dataset on fighting groups in Sierra Leone, this article analyzes the determinants of the tactics, strategies, and behaviors that warring factions employ in their relationships with noncombatants. We first describe a simple logic of extraction which we use to generate hypotheses about variation in levels of abuse across fighting units. We then show that the most important determinants of civilian abuse are internal to the structure of the faction. High levels of abuse are exhibited by warring factions that are unable to police the behavior of their members because they are more ethnically fragmented, rely on material incentives to recruit participants, and lack mechanisms for punishing indiscipline. Explanations that emphasize the importance of local community ties and contestation do not find strong support in the data.