Whether Southeast Asia more closely resembles the rose or the unicorn may be debated for some years to come. What is less doubtful is that the rose, the unicorn, or any point in between must be determined by focusing on how Southeast Asians define the region. This chapter outlines the interstate system of Southeast Asia from its historical base to its present configuration. In the traditional system, Indian religions and political concepts provided all of Southeast Asia with a modicum of cultural homogeneity, particularly among the royal elites. As a force for political legitimacy in the contemporary period, Islam has played a similarly quixotic role. At independence, the states of Southeast Asia were brought into the global diplomatic system and into such international organizations as the United Nations. As was true in the traditional Southeast Asian system, and as should be expected in any subordinate system, extraregional powers have been active in Southeast Asia throughout the post-World War II era.