Theories of International Relations and most approaches to foreign policy assume a world of `strong,' European-style states. Yet the majority of states are `weak' states, states that have difficulty policing their territory and providing public goods to their subjects. Policymakers react to these weak and failed states through `nation-building.' I explain why there are so many weak states. A priori, there are strong disincentives for elites and sovereigns to build strong states, because this requires them to make concessions to their subjects. The default organizational unit is a weak state. The system structure can change, however, to make war and empire more likely and more deadly, forcing sovereigns and their subjects to make common cause against external allies. This occurred in Europe from the late eighteenth century onwards. Yet, war and empire are unstable processes that are perceived as crises. As the scale of war and empire increases, counter-tendencies develop that seek to limit their incidence and scale. Nuclear weapons and postcolonial ideology are reactions to war and empire that transform the structure of the international system. Nuclear weapons reduce the likelihood of total interstate war; postcolonial ideology reduces the willingness of postcolonial states to engage in territorial conquest. As the risks of war and empire reduce, incentives to build states shrink. Yet, this is not a return to the past where a variety of institutional forms become feasible. Rather, because of the destructive capacity in the system, the strong state becomes even more important to maintain order, from a structural standpoint. Therefore, we are in a world where the weak state is the empirical norm, hence nation-building does not work; yet the strong state is demanded and desired, hence nation-building is attempted ever more frequently.