<![CDATA[North Korea’s nuclear aggression and China’s military modernization in East Asia and the Indo-Pacific have pushed the United States, South Korea, and Japan to strengthen Trilateral Security Cooperation (TSC). This study aims to explain the role of TSC in addressing these dual threats through the perspective of Cooperative Security. Using a qualitative descriptive-analytical method, this study analyzes secondary data from academic journals, government policy reports, trilateral agreements, defense white papers, and news reports. The analysis employs Richard Cohen and Michael Mihalka’s Cooperative Security framework, particularly The Four Rings: Individual Security, Collective Security, Collective Defense, and Promoting Stability. The results show that TSC serves as a framework for defense integration by shifting fragmented bilateral cooperation toward interdependent cooperation with a division of functional roles: South Korea focuses on ground combat defense, Japan provides maritime logistics and military bases, and the United States acts as the guarantor of nuclear deterrence. The novelty of this study lies in applying the Four Rings framework to systematically explain the functional division within TSC. Practically, the findings offer a reference for policymakers in designing coordinated defense mechanisms and transparent crisis management. The study contributes to the literature on cooperative security and alliance politics by showing how minilateral cooperation can maintain regional stability without a formal joint defense treaty.]]>
