South Korea: U.S.: Japan

An international relations topic covering the security, economic, and diplomatic cooperation framework among South Korea, the United States, and Japan, along with its historical development and latest

South Korea–U.S.–Japan

Overview

South Korea–U.S.–Japan (Korean: 한미일, Han-Mi-Il) is an international politics concept referring collectively to the diplomatic, security, and economic cooperation relations among the three countries: South Korea, the United States, and Japan. As a core axis that has shaped the security order of East Asia since the Cold War era, its level of institutionalized cooperation has risen markedly in the 2020s amid strategic environmental changes such as North Korea's nuclear and missile threats and China's rise. In particular, with the August 2023 Camp David trilateral summit as a turning point, it is assessed to have been elevated from merely the sum of bilateral relations to a 'trilateral cooperation system.'

Main Content

Historical Background

The origins of South Korea–U.S.–Japan cooperation date back to the Cold War. Through the 1951 San Francisco Peace Treaty, the 1953 Mutual Defense Treaty between the Republic of Korea and the United States, and the 1960 U.S.–Japan Security Treaty, the United States built separate alliance systems with South Korea and Japan respectively. However, because South Korea and Japan have long been in conflict over colonial rule and postwar reparations, trilateral cooperation remained for a long time in a 'hub-and-spoke' structure with the United States at the apex. After the 2010 sinking of the Cheonan and the shelling of Yeonpyeong Island, discussions on military intelligence cooperation between South Korea and Japan began, and with the signing of the 2016 General Security of Military Information Agreement (GSOMIA) between South Korea and Japan, bilateral cooperation was set in motion.

Security Cooperation

The security domain is a core axis of South Korea–U.S.–Japan cooperation. The three countries have regularized maritime missile defense exercises and anti-submarine warfare exercises in response to North Korea's ballistic missile launches, and since 2023 have begun operating a real-time system for sharing North Korean missile warning information. In addition, they have regularized trilateral defense ministers' meetings and meetings at the level of the joint chiefs of staff, strengthening the institutional foundation for military cooperation. However, both South Korea and Japan take a cautious position on developing this into a multilateral alliance at the level of a collective defense treaty.

Economic and Technological Cooperation

In the economic sphere, cooperation on advanced technology supply chains in areas such as semiconductors, batteries, and artificial intelligence has come to the fore. Following U.S. semiconductor export controls against China, South Korea and Japan occupy important positions in the supply chains for core materials, equipment, and components, and discussions on trilateral technology cooperation and supply chain stabilization have expanded. The three countries also share common interests within new economic cooperation frameworks such as the Indo-Pacific Economic Framework (IPEF).

Camp David Summit

The South Korea–U.S.–Japan summit held at Camp David in Maryland, United States, in August 2023 is regarded as a historic turning point. The three leaders adopted a joint statement containing the 'Camp David Principles' and the 'Spirit of Trilateral Cooperation,' and agreed on holding annual summits, establishing a trilateral hotline, regularizing joint military exercises, cybersecurity cooperation, and supply chain cooperation. This signifies that trilateral cooperation has entered an institutionalization phase beyond the political interests of any particular administration.

Limitations and Sources of Conflict

South Korea–U.S.–Japan cooperation also carries structural vulnerabilities. Historical disputes between South Korea and Japan (compensation for forced labor, sovereignty over Dokdo, textbook issues, etc.) are variables that could weaken the momentum for cooperation at any time. In addition, both South Korea and Japan have high economic interdependence with China, creating a structural dilemma that makes it difficult to fully align with the U.S.-led policy of containing China. The fact that the intensity of cooperation may vary depending on political changes within the United States is also a factor of uncertainty.

Latest Trends

As of 2024–2025, South Korea–U.S.–Japan cooperation is showing progress in both directions: institutionalization and substantive cooperation. The three countries have regularized annual summits and foreign and defense ministers' meetings, and are fully operating real-time sharing of North Korean missile warning information. In 2024, numerous trilateral joint military exercises were conducted, and trilateral working groups for responding to cyber threats and protecting advanced technologies were also launched.

Meanwhile, South Korea's political upheaval in late 2024 and the possibility of a change in the U.S. administration's foreign policy stance in 2025 are acting as new variables for the direction of trilateral cooperation. In particular, as U.S. trade pressure and demands for increased defense cost-sharing interlock with Japan's moves to strengthen its own independent security capabilities, analyses suggest that trilateral cooperation has entered a phase of readjustment between a 'values alliance' and 'interest-based diplomacy.' The deepening military cooperation between North Korea and Russia and rising tensions in the Taiwan Strait are also acting as factors that heighten the strategic necessity of South Korea–U.S.–Japan cooperation while simultaneously increasing the burden on each country.