The absence of a regional military alliance in Asia, and the related tendency of Asian regional institutions to avoid multilateral defence cooperation constitute a key puzzle of Asian regional order. Available theoretical explanations of this puzzle tend to focus heavily on the US role, either the nature and extent of US power, or its perceptions of collective identity. Challenging this, this paper offers a normative explanation. The absence of a "NATO in Asia", argues this paper, is explained by a norm against collective defence which emerged and evolved through early post–war regional interactions. These interactions, which have been ignored in the theoretical literature on international organization, were shaped by the interplay of the ideas of key local agents, and the evolving global norm of non–intervention. The paper's investigation into the normative origins Asian multilateralism contributes to the theoretical literature on the diffusion of sovereignty norms in the international system. International relations scholars generally assume that the "history of sovereignty is largely the history of Westphalia's geographic extension," but ignore the crucial agency of local actors in the developing world in translating the idea of sovereignty into norms of conduct in a regional setting. This article shows how regional interactions in early post–War Asia that led to a regional norm against collective defence, also helped to strengthen the global norm of non–intervention, and shaped subsequent regional institutions in Asia. In this process, Asian interactions made a distinctive contribution to the evolution of post–war international order, which has been seldom acknowledged, much less analyzed, by scholars of international relations.