The European Middle Ages have recently attracted the attention of international relations (IR) scholars as a “testing-ground” for established IR theories. Neorealists, historicizing neorealists, and constructivists dispute the meanings of medieval anarchy and hierarchy in the absence of sovereignty. On the basis of a detailed critique of these approaches, I offer a historically informed and theoretically controlled interpretation of medieval geopolitics revolving around contested social property relations. My interpretation is meta-theoretically guided by dialectical principles. Lordships are the constitutive units of medieval authority, combining economic and political powers and assigning contradictory forms of rationality to their major agents, lords, and peasants. Interlordly competition over land and labor translates directly into distinct forms of geopolitical relations, generating a culture of war. Against this background, I clarify the specific meanings of the medieval “state,” territoriality, frontiers, peace, war, anarchy, and hierarchy before drawing out the wider implications of changing social property forms for IR theory.