The Peace of Westphalia, which ended the Thirty Years' War in 1648, is generally understood as a critical moment in the development of the modern international system composed of sovereign states each with exclusive authority within its own geographic boundaries. The Westphalian sovereign state model, based on the principles of autonomy, territory, mutual recognition and control, offers a simple, arresting, and elegant image. It orders the minds of policymakers. It is an analytic assumption for neo-realism and neo-liberal institutionalism. It is an empirical regularity for various sociological and constructivist theories of international politics. It is a benchmark for observers who claim an erosion of sovereignty in the contemporary world.