This paper analyzes the evolution of regionalism in East Asia and derives implications for the role and influence of the United States and Japan in the region. In the introductory section, this paper identifies three major driving forces and three factors that acted as a brake throughout the evolutionary process of regionalism. Section 1 analyses the regionalization in East Asia to understand the characteristics of the second driving force, intra-regional economic interdependence, which has had a consistent and ever-increasing impact on the shape and nature of the regionalism. Section 2 reveals changing impacts of different factors through an overview of interactions of the driving forces and obstacles through the four distinct periods identified in the introductory section. The concluding section deals with the interaction between the regionalism and the influence of the United States, Japan and China on it.