This article examines the origins of Southeast Asia as a region, including the extent to which regularized interstate relations existed throughout the region prior to World War II. In addition, the article examines the origins of Southeast Asia’s recognition as a region together with notions of cultural homogeneity and identity. At one level, these interdependent variables are relevant to the analysis because much of the scholarly literature has utilized some combination of the three—together with notions of geographical propinquity, interdependence, and a capacity to refract the power of the international system—as a basis to substantiate the claim that a region exists, whether that be Southeast Asia or elsewhere. However, notions of culture and identity are also important because they represent a natural outcome of long-term interaction and interdependence. Consequently, the article finds that the origin of Southeast Asia, as a distinct region, most significantly stemmed from the strategic dynamics of the Second World War. Moreover, the contemporary nature of Southeast Asia’s construction as a region has been a consequence of the region’s cultural, ethnic, and religious diversity; an associated absence of a regional identity; and a lack of regionwide and regularized interstate relations until independence from colonialism.