While case-based narratives from civil wars often stress the ethnic dimension of civilian atrocities, cross-national studies have found limited evidence in support of such contentions. Addressing this debate, we argue that warring actors often use ethnic affiliation to identify groups of suspected enemy supporters when individual wartime affiliations are not known. Since warring actors depend on their civilian constituencies for support, collective targeting of the enemy’s co-ethnics becomes a strategy for weakening the enemy’s capacity. Armed actors are thus more likely to engage in civilian abuse in areas where the enemy’s ethnic constituency resides. To examine this argument, we combine new georeferenced event data on violence against civilians in African conflicts, 1989–2009, with spatial data on the location of the warring actors’ ethnic constituencies. The analysis shows that the number of civilians killed by both governments and rebel groups is higher in areas inhabited by the enemy’s ethnic constituency.