As of 2025, both East Asia and the world stand at a turning point in the postwar order that has underpinned peace and prosperity. This shift is driven not only by China’s military expansion, growing political influence, and economic standstill but also by North Korea’s nuclear missile development, the strengthening ties between Russia and North Korea, and the potential shifts in East Asia policy under the Trump admіnistration in the United States.

At the outset of this project, we explored how Japan and South Korea could build multilayered cooperation amidst an accelerating U.S.-China rivalry, economic restructuring to mitigate risks (de-risking and diversification), and the need to reassess traditional ties with China. However, as global conditions have deteriorated further and the international orders undergo an even deeper restructuring, Japan and South Korea are now compelled to enhance their cooperation at a fundamental level.

The traditional patterns of inter-state relations are rapidly becoming obsolete, creating uncertainty about the future role of alliances and regional cooperation. Meanwhile, authoritarian states are gaining strength and attracting more partners, while global challenges—such as global health governance, climate crisis, and the management of advanced technologies—demand international collaboration and dialogue. Amid the disintegration of the existing orders and weakening international cooperation, we find ourselves viewing the future with increasing anxiety and uncertainty.

In this new era, what remains essential is the strengthening and expansion of partnerships with nations that share similar perspectives on order and values. It is imperative to reevaluate the values and order we aspire to uphold, identify like-minded partners, and engage in dialogue and cooperation to realize these shared goals.

Japan and South Korea have not historically been seen as easy partners due to Japan’s historical burdens concerning the Korean Peninsula and broader Asia. While postwar Japanese society has made efforts to confront and reflect on its past, sporadic contradictory statements and actions by the Japanese government and politicians have hindered trust-building with South Korean civil society. Nonetheless, changes are evident in South Korean public perceptions of Japan, influenced by economic growth and a stronger sense of moral standing (Ogura 2024). Moreover, the rapidly deteriorating international environment has heightened the recognition of the necessity for security cooperation between the two nations. This shift has been facilitated by strong U.S. mediation, as both Japan and South Korea share the United States as their sole treaty ally. The acknowledgment of mutual necessity for pragmatic security cooperation has indeed expanded.

Beyond security, Japan and South Korea share many fundamental values in the international order. Both nations support liberal political systеms, human rights promotion, and international cooperation centered on the United Nations systеm and regional collaboration. Japan has long advocated for a Free and Open Indo-Pacific and a liberal international order, while South Korea has not only clarified its Indo-Pacific policy in recent years but has also significantly expanded its role in regional and global cooperation. This shift has strengthened South Korea’s position on the international stage.

Japan and South Korea also share key strategic interests in upholding a free trade systеm. However, the primary external forces shaping these challenges and potential solutions are China and the United States. As a trade-dependent nation, South Korea remains highly sensitive to economic and trade risks while also exhibiting skepticism toward U.S. policies. Likewise, Japan has a strong stake in preserving the stability of the free trade systеm. Beyond security concerns, Japan and South Korea have significant opportunities for economic cooperation, particularly in areas such as economic security, supply chain resilience, countering economic coercion, and reinforcing the WTO systеm.

Public opinion surveys indicate a growing understanding of Japan-South Korea relations. A joint survey by Yomiuri Shimbun and the Korea Times highlights South Korean caution regarding Japan’s acquisition of counterstrike capabilities while showing strong support for trilateral Japan-U.S.-South Korea cooperation. Additionally, an EAI-Genron NPO survey (2023) found that 59% of South Korean respondents favored issuing a joint declaration marking the 25th anniversary of the Obuchi-Kim summit.

There is a hypothesis that increased cultural exchanges will positively impact political relations; however, reality suggests a more complex dynamic (Kobari and Onuki 2024). Nevertheless, shifts in societal and international conditions are shaping perceptions of each other’s countries, and positive cultural interactions encourage travel and dialogue. As generational changes continue, the expansion of engagement channels between the two societies holds promise for improved Japan-South Korea relations.

A broader analysis of South Korean public opinion suggests that perceptions of the U.S. are not deeply polarized, support for the U.S.-South Korea alliance remains relatively stable, and there is a high level of understanding of universal values. At the same time, concerns about being drawn into U.S. East Asia policy have been rising. While the United States has played a key role in mediating Japan-South Korea relations in recent years, future diplomatic developments may require Japan and South Korea to strengthen their cooperation independently, without relying on a mediator.

Japan and South Korea, which are beginning to share common interests and a broader worldview, are gradually deepening their understanding of each other as partners—not only at the governmental level but also among their citizens. However, much work remains to be done.

How, then, can Japan and South Korea strengthen their partnership? The key may lie in deepening the recognition of each other’s reliability (Sahashi 2024). By fostering a stronger awareness that cooperation is indispensable for addressing shared challenges, both nations can develop a more proactive approach to partnership-building. These challenges may include economic issues, the rapidly evolving security environment, or global concerns. Prioritizing key issues where collaboration is essential and establishing institutional frameworks to facilitate cooperation will help forge an enduring partnership. This project aims to elucidate what such a multifaceted partnership should look like in concrete terms.

This project included the participation of seven mid-career researchers from Japan and South Korea, along with three senior scholars who attended two workshops to refine research papers. These scholars specialize in a wide range of fields, including traditional security, economic security, global governance, and China studies. Their research examines the necessity of strengthening Japan-South Korea cooperation, how their national objectives align, and how much synergy could be generated through collaboration.

Each of the research papers in this project provides a detailed assessment of specific issue areas, outlining the current landscape and the necessity for Japan-South Korea cooperation. Here, we introduce the key recommendations derived from these analyses. Importantly, these recommendations stress that they are not limited to bilateral relations between Japan and South Korea but also contribute to regional and global stability, aligning with the interests and values of both nations. While facing up to historical issues and expanding institutional frameworks for bilateral exchanges remain crucial, what is essential for a future-oriented Japan-South Korea relationship is concrete and proactive cooperation.

Economic Security and Technological Cooperation: Junghwan Lee analyzes the past economic cooperation between Japan and South Korea and emphasizes the necessity of establishing global norms in the domains of cybersecurity and AI. Seungjoo Lee examines the challenges that Japan and South Korea will face if the U.S. government moves forward with economic decoupling from China, proposing that cooperation in LNG development and procurement, as well as supply chain restructuring, be leveraged to engage the U.S. in shaping an appropriate approach toward China. Masaaki Yatsuzuka argues for Japan-South Korea collaboration in economic security intelligence and suggests that this partnership be used as a foundation for dealing with the U.S. He also proposes the introduction of military-grade unmanned reconnaissance drones as an entry point for advancing multilateral crisis management mechanisms.

Security and Geopolitical Challenges: Jiyoung Ko highlights the importance of Japan-South Korea cooperation in North Korea’s denuclearization and nuclear risk reduction. While both countries conduct bilateral discussions on extended deterrence with the U.S., she proposes that Japan, South Korea, and the U.S. engage in trilateral nuclear risk discussions. Madoka Fukuda points out the potential linkage between a Taiwan crisis and a contingency on the Korean Peninsula, urging South Korea to reassess its strategic concepts and operational plans in coordination with the U.S. and Japan, including the consideration of joint exercises. Jae Jeok Park recommends strengthening Japan-South Korea maritime security cooperation through satellite surveillance, joint patrols, and infrastructure development in Southeast Asia and the South Pacific. He also highlights the potential for collaboration in the Maintenance, Repair, and Overhaul (MRO) of U.S. naval ships.

Global Agenda and Functional Cooperation: Lastly, Kayo Takuma identifies weaknesses in regional health cooperation in Asia and recommends enhancing human resource development in Southeast Asia, deepening collaboration among the Japan-U.S.-South Korea Centers for Disease Control and Prevention, advancing Japan-South Korea cooperation in vaccine development, and expanding support for Africa’s public health initiatives.

This research underscores that Japan and South Korea must not only strengthen bilateral mechanisms but also actively contribute to regional and global stability through collaboration in multilateral frameworks. This project serves as just the first step in an evolving effort to build a robust, future-oriented Japan-South Korea partnership. Future discussions must also explore expanding institutionalized cooperation, public diplomacy efforts, and comprehensive policy coordination in key geopolitical and economic areas to ensure lasting and impactful collaboration. ■

References

Kobari, Susumu, and Tomoko Onuki. 2024. A Vision for the Future of Japan and South Korea: Passion for Culture and Diplomatic Gaps. Tokyo: Shueisha.

Ogura, Kizo. 2024. “What Does Japan Mean to South Koreans?” In Connecting with South Korea, edited by Yuki Asaba. Tokyo: Yuhikaku. 273-294.

Sahashi, Ryo. 2024. “A Vision toward Expanding the Japan-Korea Partnership in the New World Order.” IIA Pacific Report, June.

■ Ryo Sahashi is Professor of International Relations at the Institute for Advanced Studies on Asia, University of Tokyo.

■ Edited by Hansu Park, EAI Research Associate
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