This paper quantitatively analyzes the stability and breakup of nations. The tradeoff between increasing returns in the provision of public goods and the costs of greater cultural heterogeneity mediates agents’ preferences over different geographical configurations, thus determining the likelihood of secessions and unions. After calibrating the model to Europe, we identify the regions prone to secession and the countries most likely to merge. We then estimate the implied monetary gains from EU membership. As a test of the theory, we show that the model can account for the breakup of Yugoslavia and the dynamics of its disintegration. We find that economic differences between the Yugoslav republics determined the order of disintegration, but cultural differences, though small, were key to the country’s instability. The paper also provides empirical support for the use of genetic distances as a proxy for cultural heterogeneity.