This book investigates presuppositions of international order that originate in medieval theology. Part I examines rival conceptions of order that emerged out of a medieval dispute about the nature of God and the extent of his power. The theory of immanent order refers to an interconnected whole that imparts a rationally intelligible pattern of right order. The theory of imposed order refers to a contingent arrangement, established by will and artifice or the operation of an impersonal mechanism, which accommodates numerous patterns that can be made and unmade. Part II investigates the assimilation of medieval theological ideas in the thought of three emblematic thinkers: Martin Luther, Hugo Grotius, and Thomas Hobbes. This challenges the narrative of progressive secularization characterizing most international thought. Particular emphasis is placed on the transition from medieval to modern. The claim here is that continuity describes this transition as persuasively as the more familiar discourse of change. Part III uncovers a theological inheritance that shapes modern theories of international order. The language of system and society, as well as anarchy, balance of power, and contractual international law, reflects the intellectual commitments of nominalist theology. The conclusion explores the tension that arises from this theological inheritance in a world where God is only one of several postulates that can be called upon to fix the contingency of a constructed international order. The danger is that grounding international order in nothing more than human decision leaves what is made fundamentally indeterminate and precariously exposed to the whims of capricious power.