Some scholars have proposed partition as a way to solve ethnic civil wars. Partition theorists advocate the demographic separation of ethnic groups into different states, arguing that this is the best chance for an enduring peace. Opponents argue that partition is costly in terms of its human toll and that its advocates have yet to demonstrate its effectiveness beyond a limited number of self-selected case studies. This analysis systematically examines the outcome of partition, highlighting the centrality of demography by introducing an index that measures the degree to which a partition separates ethnic groups. This index is applied to all civil wars ending in partition from 1945 to 2004. Partitions that completely separated the warring groups did not experience a recurrence of war and low-level violence for at least five years, outperforming both partitions that did not separate ethnic groups and other ethnic war outcomes. These results challenge other studies that examine partition as a war outcome. The results also have direct implications for Iraq's civil war, postindependence Kosovo, and other ethnic civil wars.