Many regard ethnic nationalism as the main driver of the continuous remaking of the European map of states since the dawn of the 19th century, impacted by two world wars and the downfall of the Austrian, Ottoman, and Soviet empires. But even though nationalist ideology features prominently in historical accounts of territorial change, no accurate estimate of its impact on the division of the European landmass into countries exists. Following the literature, we argue that, since the 19th century, ethnic nationalism caused state borders to ever more align with ethnic borders. To empirically test this argument, we digitize a large number of maps of ethnic groups that cover Europe in repeated cross-sections since 1850. We use these data to (1) assess the relationship of ethnic geographies with state borders over time, and (2) estimate the effect of ethnic geographies on border changes, in particular at the critical junctions of the world wars and 1990. We do so by modelling the partitioning of the European landmass into states as a spatial conditional random field in which the shape of states is affected by the spatial distribution of ethnic groups. Our model allows for drawing inferences on the effects of ethnic geographies while taking into account inherent structural dependencies and geographic covariates that affect states’ geographies. Our preliminary findings indicate substantive effects of ethnic boundaries on the location and change of state borders since 1885. Our findings contribute to the understanding of ethnic nationalism and its enduring effects on struggles over nationhood.