Much of today's ethnic homogeneity in European states was historically produced by governments that ethnically "cleansed'" their territories. Despite the importance of this violent historical transformation, we lack systematic evidence of the conditions under which groups became targets of forced homogenization. We argue that rising nationalism in the 19th century threatened multi-ethnic states with secessionism and irredentism. To preempt the loss of territory, states turned to "right-peopling," violent ethnic homogenization through displacement and genocide. Non-dominant groups were most likely to be homogenized where the risk of territorial conflict was highest. This was the case especially where groups straddled borders and where past border changes increased the potential for revisionist nationalism. Using new spatial data on Europe's ethnic geography from 1886 to the present, we find support for our arguments.