Can US-Japan-South Korea Trilateralism Endure?

In recent years, the United States, Japan, and South Korea have moved to institutionalize trilateral relations in response to mounting regional security challenges and global supply chain vulnerabilities. What once relied on summit diplomacy has since evolved into a more structured and durable framework for coordination.

That trajectory formed the backdrop for a February 25 discussion hosted by the US-ROK-Japan Trilateral Next-Gen Study Group, where experts assessed whether the current momentum can be sustained amid shifting strategic priorities and domestic political pressures.

When the Hub Falters

Zach Cooper, a senior fellow at the American Enterprise Institute, noted that recent leadership transitions in Seoul and Tokyo could have derailed progress. "If you had told me a year ago that we would have Prime Minister Takaichi and President Lee," he said, "my guess was that trilateral cooperation was non-existent."

But instead, "we're seeing the exact opposite." With a strong conservative leader in Japan and a strong progressive in South Korea, he added, there may be real openings for trilateral cooperation.

Still, Cooper cautioned that several underlying dynamics could make the next year "a little tricky."

One challenge is the relative deprioritization of North Korea in Washington's foreign policy focus. Historically, North Korean missile tests and nuclear advances have served as the strongest catalyst for trilateral unity. When Pyongyang dominates the agenda, political differences between Seoul and Tokyo tend to narrow.

"At least from an American standpoint right now, North Korea doesn't even seem to be a priority at all," Cooper said, noting its absence in the Trump administration's National Security Strategy. 

He also questioned whether Washington will continue to invest the same degree of leader-level energy as in previous years. In the past, the US often acted as the convening force, working to "pull all three players together" during sensitive moments. That kind of direct involvement, Cooper suggested, may not be as pronounced.

With US attention divided among Ukraine, the Middle East, Taiwan, and domestic economic concerns, East Asia competes within a crowded global portfolio. Cooper framed the broader issue in systemic terms: how much can the traditional "hub-and-spoke" alliance model endure if the hub is less active — or, in some cases, perceived as impeding cooperation?

Given these realities and hemmed in by three nuclear-armed states, South Korea may be compelled to "think seriously about its own nuclear options," Cooper added, warning that such a move could trigger a broader regional arms race.

Deepening Defense Ties

Former South Korean Army Lt General Chun In-Bum focused less on diplomatic choreography and more on operational substance.

"I would love to see defense industry cooperation between the Japanese and the Koreans," Chun said, adding that trilateralism should now be defined by deeper defense coordination between the two East Asian neighbors.

He pointed in particular to cooperation on advanced stealth aircraft, including the F-35s, and related maintenance, repair, and overhaul. South Korea is planning additional acquisitions, while Japan has invested heavily in supporting capabilities and infrastructure.

Closer coordination in parts supply and maintenance could, Chun argued, create structural interdependence that strengthens deterrence and embeds cooperation beyond political cycles.

Beyond industrial, Chun advocated significantly expanding officer exchanges between the two militaries, describing current levels as minimal. Increasing those exchanges, he said, would multiply habits of coordination that are "non-aggressive, but binding and building."

Capability Before Symbolism

At the same time, Chun raised concerns about domestic political debates in South Korea that could complicate alliance cohesion. He pointed to the Unification Ministry's recent criticism of the United Nations Command as potentially damaging.

"The devil's in the details," Chun said, emphasizing that rhetorical challenges to longstanding alliance structures can generate uncertainty among partners.

He also addressed the ongoing debate over wartime operational control (OPCON) transition — the transfer of wartime command authority from the United States to South Korea.

While supportive of South Korea assuming greater responsibility, Chun said, "capability is not only about hardware, software, but the will of the people." 

Demographic decline, shortened conscription periods, and evolving public attitudes toward military service, he suggested, require careful evaluation before major structural changes are completed. In his framing, readiness and sustainability, not political messaging, should drive the pace of transition.

Integration Without Separation

These defense debates, of course, unfold within a broader evolution of US strategy in the Indo-Pacific.

Yasuyo Sakata, professor of international studies at Kanda University, cautioned against overreading narratives of American retrenchment, despite the reduced prominence of Northeast Asia in Washington's security and defense strategies. 

"In the Pacific, I don't think the US is about to give up the Pacific Ocean to China," she said.

However, Sakata acknowledged that US planning increasingly distinguishes between the Korean Peninsula and the wider Indo-Pacific theater. For Japan, that separation is not easily compartmentalized.

"Korea is in both, and for Japan, it can't be separated," she said.

As the US strategy emphasizes greater integration along the so-called first island chain, the role and flexibility of US Forces Korea may evolve. That shift, she added, requires careful coordination to ensure that Peninsula-specific deterrence and broader regional contingencies remain aligned rather than competing.

Sakata cited a recent incident in which Seoul opted out of a trilateral air exercise with the US and Japan as a reminder that domestic political constraints and timing sensitivities can still disrupt operational alignment.

Resilience as Common Ground

Alongside security concerns, the panelists stressed economic cooperation as a comparatively stable pillar of trilateral engagement.

All three countries share growing concerns over supply chain resilience, particularly in critical minerals, semiconductors, and advanced manufacturing. Efforts to diversify production networks and reduce vulnerability to economic coercion provide durable incentives for coordination.

Cooper suggested that economic security may offer common ground even if political dynamics fluctuate. Shared exposure to supply chain vulnerabilities reinforces the logic of alignment.

Sakata pointed to South Korea's interest in eventually joining the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP) as a potential avenue for deeper economic integration. While not formally on the immediate agenda, she noted signs of gradual bilateral engagement on the matter.