This study analyzes the background, process, and prospects of the concerted efforts Southeast Asian states have made for the last four decades to construct and develop regional "communities" including the Association of Southeast Asian Nations(ASEAN) and an East Asian community. It places a focus on how and why Southeast Asian leaders have attempted to expand their regionalist ideas from Southeast Asia to a broader East Asia by inviting such Northeast Asian countries as China, Japan, and Korea. As a potential theoretical contribution, an integrated approach is attempted to synthesize neo-functionalism and constructivism in analyzing and interpreting the historical process of nation- and community-building in Southeast Asia and East Asia. By bringing in theories and approaches, it tries to overcome an atheoretical tendency prevalent in the research of international relations in the regions. This paper begins with a recollection of Southeast Asian nation-building processes, which I regard as purposeful and conscious efforts by national leaders as well as popular movements to create and invent a nation from diverse and heterogeneous ethnic groups and cultures. This constructivist and circumstantialist schools of thought on nation and nationalism can also be extended and applied to an analysis of the formation of regional identity and the rise of regionalism. In this sense, Southeast Asia is an "imagined community" at the regional level as are nations at the country level. ASEAN played a pivotal role at its formative period. The constuctivist approach to community building in Southeast Asia is a partial failure in another sense, because its growth for the first two decades was slow and lacked substantial foundations such as economic integration. It is against this backdrop that Southeast Asian leaders looked to a broader East Asia, to overcome the limits and remedy the shortcomings of community building based on a narrow conception of region, Southeast Asia. The northern half of East Asia could provide them with dynamism and substance which their economies largely lack. In fact, they have been trying to supplement functionalist solutions to their basically constructivist framework. At the same time, the methods were not simply functionalist, but should be viewed as neo-functionalist as they resort mostly to intergovernmental cooperations rather than to market mechanisms. Whether ASEAN+3 develops and matures into an East Asian community shall depend primarily on the relevance of the integrated approach of constructivism and neo-functionalism, as well as its contingent modifications, to the ever changing fundamentals of East Asian politics and economies.