Under what conditions does peacekeeping reduce one-sided violence in civil wars? This article argues that local sources of violence, particularly ethnic geography, affect peacekeeping effectiveness. Existing studies focus on the features of individual missions, yet curbing one-sided violence also depends on peacekeepers’ capacity to reduce the opportunities and incentives for violence. Moving from the idea that territorial control is a function of ethnic polarization, the article posits that peacekeepers are less effective against one-sided violence where power asymmetries are large (low polarization) because they (1) create incentives for escalation against civilians and (2) are less effective at separating/monitoring combatants. The UN mission in Sierra Leone from 1997 to 2001 is examined to show that UN troops reduce one-sided violence, but their effectiveness decreases as power asymmetries grow.