This book reexamines the legitimacy of the democratic nation-state in a time of unprecedented human migration by exploring the relationship between foreignness and sovereignty in political theory. Drawing heavily on Derrida, Epstein challenges traditional theories of sovereignty as self-identicality, arguing for an “alternative understanding of foreignness as … an originary, constitutive, and ineliminable structural feature of sovereignty as such.” After arguing that both modern liberalism and conservative communitarianism tend to conflate demos with ethnos, Epstein emphasizes Thrasymachus’s central role in Plato’s Republic by meticulously unpacking the complex, contradictory relationships among guests, hosts, foreigners, citizens, friends, and enemies in that dialogue. He then turns to a multichapter examination of sovereignty in the social contract tradition, arguing that, for Hobbes, Locke, and Rousseau, political society is founded on a fear of foreignness that is to be mitigated by the sovereign’s efforts to unify its members around a common identity. Sovereignty, however, is “always already constituted” by foreignness, thereby calling for the “(non)concept” of the “foreign sovereign.” Building on Kant’s cosmopolitan right to hospitality, Derrida’s “autoimmune democracy” and “unconditional hospitality,” and Behabib’s discourse ethics, Epstein introduces the “foreign citizen,” putting the itinerant migrant at the center of any future democratic cosmopolitanism.