Debates about the future of relations among the advanced industrial countries after the Cold War hinge on theories about the sources of international political order. Realism advances the most defined—and pessimistic—answers drawing on theories of anarchy, balance, and hegemony. But these theories are not able to explain the origins and continuing stability of relations among the United States and its European and Asian partners. This article develops a theory of liberal international order that captures its major structures, institutions, and practices. Distinctive features mark postwar liberal order—co-binding security institutions, penetrated American hegemony, semi-sovereign great powers, economic openness, and civic identity. It is these multifaceted and interlocking features of Western liberal order that give it a durability and significance.