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Ⅰ. Introduction
The security cooperation between South Korea and Japan is fundamentally aimed at responding to the threats posed by North Korea and China in Northeast Asia. Additionally, there is significant room for South Korea and Japan to align their actions in the Indo-Pacific region, given that both are members of a U.S.-led security network. In the following, this paper examines the opportunities and challenges that both countries face in expanding security cooperation in the Indo-Pacific region. Opportunities arise as both countries are seeking to strengthen the U.S.-led security network amidst the intensifying strategic competition between the U.S. and China. Furthermore, a key element of both countries’ Indo-Pacific strategies/concepts is their contribution to regional maritime security.
Yet, challenges lie in the differing levels of commitment to maritime security between the two countries. Additionally, concerns persist in South Korea about the asymmetry between the U.S.-South Korea and U.S.-Japan alliances, leading to worries that the structure of South Korea-U.S.-Japan cooperation might become hierarchical. This could negatively impact not only trilateral cooperation but also bilateral cooperation between South Korea and Japan. Amidst these opportunities and challenges, this paper further presents specific areas where the two countries can collaborate.
Ⅱ. Opportunities for Security Cooperation Between South Korea and Japan
Following the Washington Declaration in April 2023, which established the Nuclear Consultation Group, and the Camp David Trilateral Summit in August 2023, U.S.-South Korea-Japan trilateral security cooperation was restored. With this restoration, South Korea and Japan (along with the U.S.) can explore ways to collaborate in ASEAN, the Pacific, and Northeast Asia. Both South Korea and Japan have pursued their respective Indo-Pacific strategies/initiatives, which closely align with the U.S. Indo-Pacific strategy. Both states view the geopolitical struggle in the Indo-Pacific as a competition between China and the broader West, with both states aiming to enhance their security cooperation with the latter.
Furthermore, both states have stepped up their efforts in marine capacity building and enhancing maritime domain awareness in the region. Intentionally or not, these initiatives contribute to countering China’s assertive maritime actions. Consequently, regional states are able to improve their Intelligence, Surveillance, and Reconnaissance (ISR) capabilities. In particular, the U.S., Japan, and Australia have individually and collaboratively supplied ISR resources and information to bolster maritime security in the Indo-Pacific. Japan’s new “Free and Open Indo-Pacific (FOIP)” plan underscores the importance of ensuring the security and safe usage of maritime and airspace domains. For instance, Japan has supported the Philippines with coastal radar systеms and Malaysia with monitoring and surveillance equipment through its Official Security Assistance (OSA) program (Park 2025).
South Korea has also enhanced its support for marine capacity building and maritime domain awareness in the Indo-Pacific region, with a particular focus on ASEAN. This includes the provision of various military and patrol assets, such as warships, airplanes, and land vehicles. Historically, South Korea has been relatively passive in engaging with security issues beyond the Korean Peninsula. However, to address these criticisms and improve its position in security-related matters, South Korea’s 2022 Indo-Pacific strategy demonstrates a clear willingness to take a more active role in resolving the Indo-Pacific’s non-traditional security challenges. Since the release of this strategy, South Korea has consistently expressed its commitment to regional security. The current government approaches maritime security from the perspective of becoming a “global pivotal power,” rather than solely accommodating U.S. strategic interests in the Indo-Pacific to strengthen the U.S.-South Korea alliance.
Under these circumstances, South Korea and Japan can further enhance bilateral security cooperation by building mutual trust and establishing a solid foundation of collaborative practices in the Indo-Pacific region. Indeed, the Indo-Pacific Dialogue among South Korea, Japan, and the U.S. was officially launched in January 2024.
Ⅲ. Challenges in Security Cooperation Between South Korea and Japan
A key challenge is the differing levels of commitment to regional maritime security. For example, Japan has actively participated in minilateral military exercises in the Asia-Pacific, including the recent S-QUAD exercises and maritime patrols involving the U.S., Japan, Australia, and the Philippines in the West Philippine Sea (Tapan 2024). These exercises have been conducted bilaterally, trilaterally, quadrilaterally, and even in coordination with some major NATO states. In contrast, South Korea has been less active in joining freedom of navigation exercises in the Indo-Pacific.
Recently, South Korea has made strong commitments to increase its contributions to regional maritime security issues. However, in its third year of pursuing the Indo-Pacific strategy, South Korea faces a pivotal question: should it actively participate in joint patrols or military exercises to demonstrate a practical commitment to regional maritime security? Proponents argue that participating in such initiatives would enhance South Korea’s positional power within the U.S.-led security network. On the other hand, critics caution against unnecessarily provoking China. In this debate, it is crucial to consider whether the U.S. will maintain the momentum of the S-Quad by continuing to work with allies and security partners through minilateral approaches. If the U.S. sustains these efforts, South Korea’s participation could be seen as both an investment in the U.S.-ROK alliance and a contribution to regional maritime security. However, if the U.S. pulls back, South Korea might need to exercise greater caution in its decision to join. Moreover, even if the Trump admіnistration continues the momentum for the S-Quad, domestic turmoil in South Korea could prevent full participation.
Second, South Korea and Japan have nuanced differences in their approaches to global partnerships. Japan places greater emphasis on India and the Global South compared to South Korea. For example, Japan has engaged in Japan-U.S.-India trilateral cooperation, Japan-India coordination in engaging with the Global South, and minilateral infrastructure investment efforts alongside India. In contrast, South Korea’s Action Plan for a Free, Peaceful, and Prosperous Indo-Pacific, released in December 2023, includes only a few plans specifically targeting India.
Third, Japan views a Taiwan contingency as a direct threat to its security, while South Korea remains more focused on the Korean Peninsula. In the event of a Taiwan contingency, South Korea is likely to provide only indirect support. Though President Yoon was intentionally vocal on regional security issues, including Taiwan, he aimed to bolster South Korea’s position within the U.S.-led security network rather than making a commitment to become involved in a Taiwan contingency. South Korea remains concerned that North Korea may exploit the situation by engaging in military provocations in the West Sea or along the border, if a Taiwan contingency occurs.
Fourth, following the third point, South Korea is concerned that the U.S. may prioritize the U.S.-Japan alliance over the U.S.-South Korea alliance, potentially creating the perception that South Korea is a second-tier U.S. ally subordinate to Japan. If this perception persists, it could hinder not only security cooperation between South Korea and Japan but also trilateral cooperation with the U.S.
Ⅳ. Key Items and Strategic Directions for Cooperation
Both states should enhance coordinated contributions to maritime security. The list of potential areas for cooperation includes satellite collaboration for maritime domain awareness in regional states such as Vietnam, participation in “Quad - x + α” military exercises, and maritime patrols (e.g., the Malacca Straits Patrol and the Sulu-Sulawesi Patrol). South Korea has participated in multilateral military exercises in the Indo-Pacific involving the U.S., Japan, Australia, and South Korea. If South Korea and Japan were to conduct bilateral military exercises involving ground forces during such events, it might draw less domestic attention. Although South Korea has restored security relations with Japan, public sentiment in South Korea remains cautious, particularly regarding bilateral military exercises involving ground forces.
Additionally, South Korea and Japan could collaborate on infrastructure projects in Southeast Asia and the South Pacific. In Northeast Asia, South Korea, the U.S., and Japan could collaborate to utilize the United Nations Command (UNC) and its UNC-Rear as primary channels for connecting U.S.-led alliances in the Indo-Pacific with NATO in Europe. The UNC is headquartered in South Korea, with rear bases located in Japan and the commander of the USFK serves as the commander of the UNC. Consequently, South Korea, Japan, and the U.S. can play pivotal roles in revitalizing the UNC.
Another key area for cooperation is the Maintenance, Repair, and Overhaul (MRO) of U.S. naval ships. Japan has been providing MRO services for the naval ships of the U.S. 7th Fleet. Recently, Japan and the U.S. have been negotiating to expand Japan’s MRO services to support not only the U.S. Navy but also the U.S. Air Force (Tanaka 2024). Meanwhile, the MRO industry in South Korea has been gaining attention, particularly after President Trump, as President-elect, highlighted it during a phone call with President Yoon. As the MRO market expands, U.S.-led MRO operations in South Korea could generate significant economic benefits. However, certain challenges remain unresolved, such as South Korea currently having only one dock available for MRO activities. This limitation has increased interest in Subic Bay in the Philippines as an alternative hub (Lee 2024). The U.S., Japan, and Australia are reportedly planning to expand the MRO capacity of Subic Bay through public-private partnerships, using it as a hub for U.S. naval operations in the Indo-Pacific. From South Korea’s perspective, participating in MRO operations at Subic Bay could provide dual benefits. First, it could offer South Korea economic advantages, and second, it would further solidify its investment in the U.S.-South Korea alliance by accommodating U.S. strategic interests in maintaining naval power in the region. South Korea and Japan should focus on discussing ways to collaborate on MRO operations to align with their shared strategic interests, rather than competing over narrow economic gains.
On the other hand, while fostering the U.S.-led security network, South Korea’s and Japan’s Indo-Pacific strategies/initiatives currently lack a strong focus on middle power-oriented minilateralism. The two states should actively promote security cooperation with or without U.S. involvement. They should also explore opportunities for collaboration with countries in Southeast Asia and the Pacific. For example, while South Korea, Japan, and Australia are already jointly undertaking development cooperation projects in Southeast Asia, these efforts could be expanded to the South Pacific, where strategic importance is increasing. Trilateral or quadrilateral cooperation with Australia and Indonesia on energy security could also be considered, with potential expansion to include the Philippines and Vietnam, both of which have shown interest in defense and energy partnerships with South Korea and Japan.
If a mini-lateral coalition consisting of key developed countries in the Indo-Pacific region—including South Korea, Japan, Australia, India, Indonesia, and Vietnam—can enhance its status within the U.S.-led security network, it may also improve its autonomy. This could prevent the U.S.-led security network from becoming solely a mechanism for containing China in the U.S.-China rivalry. Moreover, if this coalition can establish links with minilateralism in ASEAN, such as initiatives to enhance cooperation along the Mekong River, combat piracy, and share maritime intelligence, it could lay the groundwork for a multilateral security cooperation framework independent of U.S.-China competition.
Ⅴ. Conclusion
Security cooperation between South Korea and Japan is vital for addressing shared challenges in the Indo-Pacific. By leveraging opportunities and tackling challenges, the two states can contribute to a more stable and secure regional order. Building trust, enhancing bilateral initiatives, and fostering minilateral partnerships will be key to achieving these objectives.
To this end, building on the trilateral dialogue among South Korea, Japan, and the U.S., which was launched in January 2024, bilateral discussions such as a South Korea-Japan ASEAN Policy Dialogue should be explored. This would mirror the successful framework South Korea has maintained with Australia since 2021. ■
References
Lee, Peter. 2024. “Australia’s Shipbuilding Ambitions and South Korea’s Experience.” United States Studies Centre. October 31. https://www.ussc.edu.au/australias-shipbuilding-ambitions-and-south-koreas-experience (Accessed January 24, 2025)
Park, Jae Jeok. 2025. “American Coalition Building of the US-led Security Network in the Indo-Pacific: US Influence-Building Measures.” Australian Journal of International Affairs, 1-16. https://www.tandfonline.com/doi/full/10.1080/10357718.2025.2458692 (Accessed February 13, 2025)
Tanaka, Moyuru. 2024. “Navigating the Competitive Seas: U.S.-Japan Future Collaboration in the Shipbuilding Industry.” CSIS. August 13. https://www.csis.org/analysis/navigating-competitive-seas (Accessed January 24, 2025)
Tapan, Oorja. 2024. “The Squad: Adding an ‘S’ for Security.” The Diplomat. May 11. https://thediplomat.com/2024/05/the-squad-adding-an-s-for-security/ (Accessed January 24, 2025)
■ Jae Jeok Park is Associate Professor of Graduate School of International Studies (GSIS) and Underwood International College (UIC), Yonsei University.
■ Edited by Hansu Park, EAI Research Associate |