Chapter 4 traces the development of states in early modern Europe by revisiting Charles Tilly’s bellicist theory of how war and state formation interacted to reduce the number of states in Europe through persistent warfare. Focusing on state formation before the French Revolution, the chapter sets up a historical baseline that prepares the ground for our analysis of how nationalism affects state borders. Analyzing the external aspects of Tilly’s theory, we reformulate it as observable propositions that are tested systematically with geocoded data on state borders and interstate wars from 1490 through 1790. Proceeding at the systemic, state, and dyadic levels, our analysis confirms that warfare played a crucial role in the territorial expansion of European states, with power differentials increasing the chance of war, which let large states grow ever larger through conquest. Small states disappeared in the process, which in turn increased the average size of states.

In Chapter 10, we shift the attention toward the right-peopling of states within their existing borders. We test conventional constructivist expectations by flipping the causal pathway back to ethnic settlements via ethnic cleansing. This is a crucial complement to our focus on border change since even a casual reading of European states’ violent history and our analyses suggest that much of their current ethnic homogeneity resulted from ethnic cleansing. We directly link ethnic cleansing to states’ fear of being right-sized. As rising nationalism in the nineteenth century threatened multiethnic states with the secessionism and irredentism examined in the preceding chapters, some states turned to ethnic cleansing to prevent territorial losses. To test this argument, we analyze our spatial data on changes in ethnic settlement areas from 1886 to the present, which we link to episodes of ethnic cleansing. Building on Chapters 7 and 8, we find that border-crossing groups and those with a history of lost home rule were most at risk of ethnic cleansing, especially in times of interstate war. The analysis closes the circuit and shows how right-peopling and right-sizing are part of the same macrohistorical process of nationalist state transformation.

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.