In the context of the end of the Cold War and the spread of globalism, sub-regions are attracting attention as new social units of international society never before observed. In the "second wave" of regionalism that became active in the 1980s, a new regionalism, which differed qualitatively from the old regionalism, expanded globally. This "new regionalism" is characterized by multi-dimensionality, complexity, fluidity, and non-conformity, and within it we cannot overlook the fact that spaces on a new scale, such as sub-regions, are being formed in various parts of the world. The sovereign state system that has continued unbroken since the Westphalia Treaty is being transformed, and within this context, the increase in the number of sub-regions as new social units adds to the sense that we have arrived at a post-Westphalian international order. This book focuses on sub-region as a new social unit of international society. It is based on the findings obtained through meticulous fieldwork and joint studies conducted over the past 10 years by about 20 researchers, primarily from Japanese universities and Chiang Mai University, Thailand. The sub-regions described here are mostly international cross-border spaces or units in the interior of a certain region, which include: multiple states, states and parts of states, or more than two parts of states (often referred to as micro-regions). Such sub-regions have been formed in various parts of the world since the end of the Cold War. However, studies on sub-regions remain unexplored in the existing studies of regionalism. The few studies that do exist mainly focus on the economic aspects of sub-regions. In contrast, this book will specifically examine the sub-regions in Asia (especially the Mekong region and Europe) as main cases from a political science and international relations perspective, aiming to establish a new/alternative international relations by carving out a political angle of sub-region as a new social unit of international society and attempting to shift the paradigm of conventional international relations. To understand the political dimension of a sub-region, this book will mainly focus on three aspects: sub-regions and state strategies, bottom-up dimension of sub-regions, and sub-regions and borders.