Between Alliance Intermediary and Soft Balancing: Japan’s Dual Role in U.S. Diplomacy in Southeast Asia

This study examines the dual role of Japan in U.S. diplomacy in Southeast Asia: the role of "mediator in the alliance" and "soft balancing." Considering Japan's foreign policy practice in the context of U.S.-East Asian relations, the authors explore the mechanisms by which Japan finds a balance between fulfilling its allied obligations and maintaining strategic autonomy. The author describes in detail how Japan, being a weak ally in an asymmetric alliance, strengthens and legitimizes the US strategic agenda through multilateral dialogues and institutional mechanisms (such as the synergy of the Freedom and Openness and Sovereignty and Independence initiatives, as well as trilateral cooperation with the Philippines). In addition, the study examines how Japan uses "soft balancing" to limit U.S. influence and reduce the risk of direct confrontation. Thus, this study not only comprehensively highlights the theoretical details of the concepts of "allied mediation" and "soft balancing", but also illustrates their practical interaction in specific regional cases, demonstrating that Japanese diplomacy operates within a multi-level logic: externally it serves as a tool to strengthen the US agenda, and internally as a mechanism to protect its own autonomy.. In this article, literature analysis is used to further understand the meaning of soft balance through academic literature, and a case study method such as the South China Sea issue is applied to further validate this topic. The main conclusion of this study is that Japan's diplomacy in Southeast Asia combines a dual logic: external action as an "allied mediator" for the United States and internal implementation of a "soft balancing" strategy to expand autonomy. The innovation of this study lies in the fact that it shows the intertwining and intersection of these two theoretical approaches, rather than their isolated application to real politics, which contributes to a deeper understanding of Japan's unique role in the regional order. The purpose of this article is to improve analytical tools by offering a narrower interpretation of the concept of "allied mediation" in order to more accurately explain the behavior of weak allies in asymmetric alliances, while analyzing international relations from the point of view of "soft balance".