Chapter 5 addresses the puzzle of reversing state size, which is inconsistent with a Tillyan account of European history. We argue that border-change processes triggered by ethnic nationalism are the main drivers of this development. While unification nationalism increased states’ size in the nineteenth century, its effects were dominated by secessionism, which shrinks states. Irredentism, in turn, had no effect on average state size. Focusing on deviations from the nation-state ideal, we postulate that internal ethnic fragmentation leads to secession and reductions in state sizes and that the cross-border presence of dominant ethnic groups makes state expansion through unification or irredentism more likely. Conducted at the systemic and state levels in Europe and globally, our analysis exploits information at the interstate dyadic level to capture these border-change processes. We find that while nationalism exerts both integrating and disintegrating effects on states’ territories, it is the latter effect that has dominated since the twentieth century.