Why did the nation-state proliferate across the world over the past 200 years, replacing empires, kingdoms, city-states, and the like? Using a new dataset with information on 145 of today’s states from 1816 to the year they achieved nation-statehood, we test key aspects of modernization, world polity, and historical institutionalist theories. Event history analysis shows that a nation-state is more likely to emerge when a power shift allows nationalists to overthrow or absorb the established regime. Diffusion of the nation-state within an empire or among neighbors also tilts the balance of power in favor of nationalists. We find no evidence for the effects of industrialization, the advent of mass literacy, or increasingly direct rule, which are associated with the modernization theories of Gellner, Anderson, Tilly, and Hechter. Nor is the growing global hegemony of the nation-state model a good predictor of individual instances of nation-state formation, as Meyer’s world polity theory would suggest. We conclude that the global rise of the nation-state is driven by proximate and contextual political factors situated at the local and regional levels, in line with historical institutionalist arguments, rather than by domestic or global structural forces that operate over the long durée.

The existing quantitative literature on war takes the independent nation-state as the self-evident unit of analysis and largely excludes other political types from consideration. In contrast, the authors argue that the change in the institutional form of states is itself a major cause for war. The rise of empires and the global spread of the nation-state are the most important institutional transformations in the modern age. To test this hypothesis, the authors introduce a new data set that records the outbreak of war in fixed geographic territories from 1816 to 2001, independent of the political entity in control of a territory. Analysis of this data set demonstrates that wars are much more likely during and because of these two transformations. For the transformation to the modern nation-state, the authors confirm this hypothesis further with logit regressions that control for variables that have been robustly significant in previous research. The results provide support for the main mechanisms that explain this time dependency. Modern nation-states are ruled in the name of a nationally defined people, in contrast to empires, which govern to spread a faith across the world, to bring civilization to backward people, or to advance the world revolutionary cause. The institution of the nation-state thus introduces incentives for political elites to privilege members of the national majority over ethnic minorities, and for minority elites to mobilize against such political discrimination. The resulting power struggles over the ethno-national character of the state may escalate into civil wars. Interstate wars can result from attempts to protect co-nationals who are politically excluded in neighboring states. The reported research thus provides a corrective to mainstream approaches, which exclude ethnic and nationalist politics as factors that would help understanding the dynamics of war.

Quantitative scholarship on civil wars has long debated whether ethnic diversity breeds armed conflict. We go beyond this debate and show that highly diverse societies are not more conflict prone. Rather, states characterized by certain ethnopolitical configurations of power are more likely to experience violent conflict. First, armed rebellions are more likely to challenge states that exclude large portions of the population on the basis of ethnic background. Second, when a large number of competing elites share power in a segmented state, the risk of violent infighting increases. Third, incohesive states with a short history of direct rule are more likely to experience secessionist conflicts. We test these hypotheses for all independent states since 1945 using the new Ethnic Power Relations (EPR) data set. Cross-national analysis demonstrates that ethnic politics is as powerful and robust in predicting civil wars as is a country's level of economic development. Using multinomial logit regression, we show that rebellion, infighting, and secession result from high degrees of exclusion, segmentation, and incohesion, respectively. More diverse states, on the other hand, are not more likely to suffer from violent conflict.