Abstract This article advances a novel theoretical framework for explaining the emergence of international and domestic conflicts, especially in the twenty-first century. I argue that nationalism plays a major role in the rise of these conflicts. Yet, nationalism is not monolithic. I distinguish among five types of nationalism (satisfied; stateless; consolidating; irredentist; populist). The variations in the type of nationalism explain variations in peace and conflict in different parts of the world. The explanation of the variations of types of nationalism, in turn, is based on the combined effect of variations in state capacity (i.e., the functioning of state institutions) and national congruence (i.e., the congruence between national identities and state borders). Variations in these two independent variables account for both civil and international wars as well as for peaceful states and for domestic polarization. Thus, national congruence and high capacity produce satisfied nationalism and a peaceful state. In contrast, national incongruence and low capacity lead to stateless nationalism and, thus, to civil wars in failed states. High capacity and national incongruence, especially external incongruence, produce irredentist nationalism of revisionist states, leading to war-prone interstate conflicts. High capacity and declining congruence generate nationalist populism and societal polarization. Thus, the theory developed here explains the recent rise of nationalist populism (and the related domestic polarization) in quite a few democracies in comparison with other types of nationalism and the conflicts they generate.