From Momentum to architecture: Making trilateralism work for Korea, Japan, and the United States
From Momentum to Architecture: Making Trilateralism Work for Korea, Japan, and
the United States
WRITTEN BY YUN KYUNG KIM
20 August 2025
The past few years have witnessed a remarkable thaw in relations between Seoul and Tokyo, driven largely by shared regional concerns over North Korea’s growing nuclear arsenal and China’s strategic assertiveness in the Indo-Pacific. The Camp David summit in August 2023 symbolised this rapprochement, with leaders pledging to deepen cooperation across security, economic, and technological domains.
However, despite these advances, the current blueprint of regional cooperation remains fragile and overly dependent on ad hoc bilateral consultations with Washington, underscoring the long-standing model of US-centred bilateral alliances in the region. This dependence persists because many of the challenges facing Seoul and Tokyo, ranging from nuclear deterrence to supply chain resilience, are inherently regional and require US capabilities as well as commitments to manage effectively.
To secure long-term stability and amplify current momentum, South Korea and Japan must shift from parallel bilateral engagements to a formalised trilateral framework. Such a framework is not only desirable but essential for safeguarding each country’s security and strategic position in the Indo-Pacific.
A joint approach on institutionalising trilateralism would not only anchor the partnership in predictable structures — shielding it from the volatility of domestic politics — but also enhance South Korea and Japan’s collective bargaining power with Washington and signal to regional actors like China and North Korea that this is a durable strategic coalition rather than a temporary alignment of convenience.
The limits of bilateralism and why trilateralism matters
For decades, the US alliance system in Northeast Asia has been characterised by what scholars call a “hub-and-spokes” model, in which Washington serves as the central hub managing separate bilateral alliances, while its key allies — such as Seoul and Tokyo — remain largely disconnected from each other. This design offered flexibility but at the cost of coordination: historical grievances, domestic politics, and the absence of institutionalised channels meant that the two Asian allies rarely interacted beyond crisis-driven dialogues.
Recent improvements, such as intelligence-sharing agreements and joint military exercises, remain mostly reactive and contingent on political goodwill. A change of administration in either Seoul or Tokyo could easily reverse these gains, as seen in the past. Moreover, bilateralism perpetuates a zero-sum dynamic where South Korea and Japan compete for US attention and concessions, particularly in areas such as technology access or extended deterrence guarantees. Such a fragmented approach not only constrains strategic coherence but also weakens deterrence. Adversaries can exploit these gaps, confident that the absence of binding trilateral mechanisms limits the ability to mount a unified response towards them.
Korea and Japan now have an opportunity to redefine their roles — not as competing spokes but as co-architects of a trilateral framework that can anchor stability in the Indo-Pacific. For Washington, embracing this shift means sharing agenda-setting space with allies to maximise American interests and maintain primacy.
Current developments underscore these limitations. Both Seoul and Tokyo have recently clinched scaled-back trade deals with Washington: Tokyo secured tariff relief and expanded economic terms, and Seoul concluded an agreement, although still facing unresolved issues — including profit-sharing on the investment fund, defence cost-sharing, and digital trade — poised for discussion during an upcoming summit. Despite these efforts, such bilateral deals are unlikely to yield substantial or balanced benefits, as US decisions primarily serve its own economic and strategic priorities.
Instead of pursuing parallel and often competing negotiations, South Korea and Japan would be far better positioned by acting collectively through an institutionalised trilateral framework. It is not just a courtesy to allies; it directly advances Washington’s long-term strategic objectives.
First, trilateral mechanisms streamline alliance management while reinforcing US agenda-setting power. A unified framework enables more efficient coordination on export controls, critical technologies, and defence posture, reducing the risk of fragmented approaches that could dilute US influence. For Seoul and Tokyo, it provides a platform to shape alignment and reduce asymmetries in decision-making.
Second, trilateralism strengthens deterrence and sends a clear strategic signal, particularly to China and North Korea, that the partnership is a durable strategic coalition. This ensures allies collectively shape regional norms on security, technology, and economic governance in line with democratic values.
Finally, integrating the industrial capacities of South Korea and Japan, especially in shipbuilding and advanced manufacturing, can significantly amplify US capabilities. These synergies cannot be fully realised through bilateral channels alone. A trilateral approach enables integrated planning on naval production, supply chain resilience, and technology cooperation that advances shared interests more effectively.
Ultimately, institutionalising this cooperation allows a win-win strategy to leverage their combined weight and build on the foundational principles established at Camp David, transforming episodic alignment into a durable strategy.
Policy pathways
A phased approach could help trilateralism move from concept to reality. Establishing an official Trilateral Defence Council between the Ministers of Defence to oversee joint exercises, interoperability planning, and extended deterrence dialogue would be a good first step. Unlike current ad hoc drills, a permanent body at a practical level would enable long-term planning and rapid crisis coordination. For the US, this reduces the burden of bilateral management, while Korea and Japan gain confidence in shared deterrence and crisis response mechanisms that are more resilient to domestic political shifts.
Then, an Economic Security Forum focused on supply chain resilience, export control alignment, and critical technology governance could be created. Semiconductor and battery supply chains are ideal starting points, given their strategic value and mutual dependence. This forum would provide the US with more reliable allies for securing critical technologies, while Korea and Japan obtain greater leverage and predictability in trade negotiations, reducing vulnerability to sudden tariff changes or export restrictions. The collective approach also reinforces all three economies by coordinating friendshoring efforts and boosting technological competitiveness against regional challenges.
Finally, Track 1.5 dialogues between the government and private sectors, and Track 2 dialogues between academia, industry and civil society (including youth perspectives) could be institutionalised to deepen trilateral cooperation. The Young Trilateral Leaders (YTL) Summit, hosted by the US State Department, was first held in Busan last year and convened again in Osaka this year, exemplifying the potential of multi-level engagement. Sustaining and expanding such initiatives will ensure trilateralism extends beyond elite-driven security agendas, incorporates societal concerns such as digital governance and climate resilience, and builds a broad foundation of public support across all three nations, making cooperation more sustainable and less turbulent to political turnover.
Crucially, these mechanisms should prioritise transparency and inclusivity, distinguishing them from exclusive blocs. By linking initiatives to broader regional platforms such as the Quad or ASEAN Regional Forum (ARF), the three partners can project a cooperative, rules-based approach that benefits the region as a whole while maximising the strategic interests of each country involved.
From momentum to architecture
The Camp David summit demonstrated what is possible when political will aligns. Yet, momentum alone cannot guarantee permanence. Without institutional scaffolding, trilateral cooperation risks reverting to its historical pattern of episodic engagement. The strategic environment — marked by systemic rivalry, economic coercion, and technological bifurcation — demands more than ad hoc summits.
Korea and Japan now have an opportunity to redefine their roles — not as competing spokes but as co-architects of a trilateral framework that can anchor stability in the Indo-Pacific. For Washington, embracing this shift means sharing agenda-setting space with allies to maximise American interests and maintain primacy. For Seoul and Tokyo, it means accepting that the cost of influence is deeper interdependence. The alternative is to let the promise of 2023 become another missed opportunity in a region that can ill afford strategic drift.
DISCLAIMER: All views expressed are those of the writer and do not necessarily represent that of the 9DASHLINE.com platform.
Author biography
Yun Kyung Kim is an international affairs and public diplomacy professional with expertise in cross-cultural engagement and strategic communication. She recently earned an MA in International and Intercultural Communication from American University and is currently participating in the 2025 Emerging Leaders Forum at the Indo-Pacific Studies Center. She is also an alumna of the 2025 Young Trilateral Leaders (YTL) Summit organised by the US State Department. Image credits : Flickr/Chairman of the Joint Chiefs of Staff.