Contemporary Europe largely consists of ethnically homogenous nation-states. Though often treated as exogenous, the current ethnic map is the product of a long history of ethnic homogenization that often involved considerable violence. Despite the importance of this historical transformation, it has rarely been studied systematically. As a result, we lack a clear understanding of the conditions under which states resort to forced homogenization, and which groups are likely to become the target of such efforts. This project conducts a geospatial analysis of ‘right-peopling’ that uses new data on historical ethnic geography. We argue that the rise of nationalism in the 19th century made it increasingly challenging for states to rule diverse populations, which created incentives to homogenize their people through assimilation, displacement, and genocide. However, homogenization efforts also carried the risk of violent backlash of target populations against the state and members of its dominant group. From this logic, we derive the following two expectations: First, ethnic cleansing and assimilation were more likely in states where one group was large enough to claim titular status. Second, small and geographically dispersed groups were most vulnerable to forced homogenization as they lacked the capacity to mobilize. To test these claims, we introduce a new spatial dataset of ethnic groups that covers Europe from 1850 to the present. We combined information from over 80 historical ethnic maps to reduce potential distortions due to imprecision and bias and relied on secondary sources to trace changes in ethnic geography over time. The results of our analysis will contribute to a better understanding of endogenous nation-building processes that shaped the present ethnic map and contemporary cases of ethnic cleansing, such as that of the Rohingya in Myanmar and Uyghurs in China.