Expanding our analytical scope beyond states’ aggregate size, Chapter 6 offers a more general analysis of border change that fully endogenizes the shape of states since the late nineteenth century. Taking the partitioning of the European landmass into states as the main outcome, we directly test the overarching argument that nationalism creates pressures to redraw political borders along ethnic lines, ultimately making states more congruent with ethnic groups. Based on an innovative probabilistic spatial partition model, we conceive of state territories as partitions of a planar spatial graph. Encoding data derived from historical ethnic maps on a graph as the main explanatory factor, the analysis shows that ethnic boundaries increased the conditional probability that the two locations they separate are, or will become, divided by a state border. As before, we substantiate the finding that secession is an important mechanism driving this result. Moving beyond Europe, we find similar dynamics of border change in Asia, but not in Africa or the Americas.