In this dissertation I develop a theory that seeks to account for the variation in order present across regions. I propose that the observed variation in regional order in the international system is rooted in the domestic politics of region members. Unlike other attempts at explaining regional order, I account for domestic politics in the political capacity of member states. Measured as the relative ability of states to extract resources from their domestic societies, political capacity provides a measure of institutional and cultural constraints upon the ability of states to engage in costly foreign policies, such as conflict. The more extensive these constraints, the more likely a state will engage in cooperative behavior, resulting in more extensive regional institutions or trade interdependence. I show that regions comprised of high capacity democracies, like Europe, are highly cooperative, while those comprised of high capacity autocracies, like the Middle East, are more conflictual. The more cooperative the region, the greater the degree of interdependence and institutional architecture that will emerge. Finally, because the presence of regional order is contingent upon the domestic characteristics of constituent states, I develop a novel means of identifying regions for the proper measurement and identification of regional variables of interest. Using an opportunity and willingness framework, I define regions as stable geographic spaces of interacting states behaving uniquely from the broader international system. The resulting empirical analysis is a new dataset that provides not only a necessary means of case selection for the regional level variables included in this dissertation, but a specification of regions broadly applicable to regionalist research.