In the last week of January, U.S. Under Secretary of Defense for Policy Elbridge Colby visited South Korea and Japan, Washington’s two most important allies in the Indo-Pacific. His trip came directly after the release of the Pentagon’s National Defense Strategy (NDS), which emphasized “deterrence by denial along the First Island Chain (FIC)” and a renewed focus on “Peace through Strength,” calling the United States’ allies and partners to take greater responsibility for their own defense.
As China’s revisionist expansion destabilizes the regional order and heightens the risk of a Taiwan contingency, expanding Japan-South Korea-U.S. trilateral cooperation beyond the Korean Peninsula will be essential to demonstrate the collective resolve needed to uphold the balance of power emphasized by the NDS. As such, Colby’s visit to both South Korea and Japan was not merely symbolic. Rather, his trip served to reinforce the Trump administration’s Indo-Pacific vision, in which allies’ shoulder “their fair share” of collective defense.
A collective defense for the Indo-Pacific requires not only credible militaries but also the political will to accept shared strategic risks. A clear demonstration of capability and resolve by Seoul and Tokyo can extend their deterrence without overcommitting U.S forces. With strong economies and rapidly modernizing conventional military capabilities, South Korea and Japan are uniquely positioned to anchor Washington’s efforts to deter China’s attempt to gain regional hegemony – provided they continue to translate coordination into operational practice. Thus far, both Seoul and Tokyo are echoing this priority and signaling their resolve to work closely with Washington.
At last October’s South Korea-U.S. Summit, U.S. President Donald Trump welcomed President Lee Jae-myung’s commitments to strengthen South Korea’s military capabilities, raise defense spending to 3.5 percent of GDP, attain the capacity to hold operational control of its forces, and pursue greater strategic autonomy. Japan has similarly advanced defense reforms, with Defense Minister Koizumi Shinjiro outlining plans to revise the National Security Strategy, deepen Japan-U.S. command-and-control integration, and expand alliance training in Japan’s southwestern region in support of a Free and Open Indo-Pacific, following Japan’s achievement of its initial defense spending goal.
Beyond these parallel reforms, deepening security ties between Seoul and Tokyo signal a mutual interest in closer cooperation and growing autonomy within their alliances with Washington. Just ahead of Koizumi’s meeting with South Korea’s Defense Minister Ahn Gyu-back in Yokosuka, Japan’s Self-Defense Forces (SDF) provided refueling and logistical support to a South Korean Air Force C-130 and its Black Eagles aerobatic team, marking the first such logistical support between the two militaries following Tokyo’s earlier refusal to do so. Substantively, both ministers agreed to expand people-to-people and troop exchanges between their forces and to resume their search-and-rescue exercises (SAREX), paving the way for Seoul and Tokyo to move past historical grievances that have long constrained efforts advancing their military cooperation.
These evolving defense postures in Seoul and Tokyo align with the core theory of the NDS’s “flexible realism” – self-reliant allies and partners who contribute equitably and strategically coordinate around shared interests. Colby articulated the logic in remarks at the Sejong Institute in Seoul, where he praised the country as a “model ally” and lauded its efforts in defense spending. Additionally, Colby noted that deterrence by denial emphasizes making aggression infeasible by denying adversaries quick or decisive military gains, and that the focus should be on a modernized force along the FIC that is in shared pursuit of peace and stability.
For Seoul and Tokyo, this means Washington expects them to continue investing in resilient, interoperable, and forward-positioned conventional capabilities that directly contribute to a stable balance of power and deny China regional hegemony in the Indo-Pacific. Even while remaining their security guarantor, Washington can enable Japan and South Korea’s domestic development, strengthen allied defense industrial bases, and integrate ISR capabilities that reinforce early-warning and information-sharing architectures. Therefore, it is important that Seoul, Tokyo, and Washington align their national security interests to give flexibility to both alliances.
While the deterrence-by-denial strategy hinges on the unequivocal integration of South Korea and Japan into U.S. alliance structures, continued ambiguity about whether Washington would defend Taiwan risks delaying the rapid, decisive action such a strategy requires. Given Taiwan’s position at the center of the FIC, denying China “quick or decisive gains through military force” inherently implies allied intervention in a Taiwan contingency.
Although Seoul and Tokyo’s pursuit of greater self-reliance signals progress toward burden-sharing, the ambiguity surrounding support for U.S. action in a Taiwan defense scenario undermines the NDS’s denial strategy by reducing the speed, cohesion, and legitimacy required for an effective trilateral response should deterrence fail.
Reinvigorated Japan-South Korea-U.S. trilateral security cooperation offers Washington a practical way to translate aligned strategic priorities into operational readiness under a deterrence-by-denial framework along the FIC, even amid sensitivities surrounding Taiwan. Because the NDS omits Taiwan as a central element, Washington must further operationalize trilateral cooperation through the Trilateral Security Cooperation Framework, including standing dialogues, exercises such as Freedom Edge, and enhanced information-sharing. Doing so would align regional objectives, reduce political friction, and signal to Seoul and Tokyo that a free and open Indo-Pacific is inseparable from stability in the Taiwan Strait.
The maturation of Japan-South Korea-U.S. trilateral security cooperation depends on coordinated efforts that reinforce both operational effectiveness and political legitimacy in pursuit of Colby’s and the National Defense Strategy’s vision of a “decent peace” with Beijing. As Seoul and Tokyo move toward deeper military cooperation, Washington gains greater flexibility to manage its alliance architecture in the Indo-Pacific and encourage its allies to move beyond their current roles in support of a collective defense.
In this sense, a successful deterrence-by-denial strategy along the FIC hinges on legitimacy – anchored in Seoul’s regional buy-in, Tokyo’s sustained resolve, and Washington’s continued leadership.
Source:
thediplomat.com
