[Global NK Commentary] Fractured Trilateral Cooperation?: South Korea, U.S., and Japan's Threat Perceptions and the North Korea Problem

■ Go to the original text on Global NK Zoom&Connect

In August 2026, President Trump, citing high costs and a desire to improve relations with North Korea, directed the U.S. Department of Defense to significantly scale back combined military exercises with South Korea. Posting his grievances on social media, President Trump criticized the exercises as expensive and condemned South Korea for not supporting U.S. actions regarding Iran. The subsequent bilateral combined exercises were shortened in duration, reduced in scale, and concluded several days ahead of schedule. The ROK-U.S. combined exercises have long served as the most visible symbol of America’s security commitment to South Korea. This abrupt reduction, therefore, signifies more than a simple budgetary adjustment. It was a deliberate signal that the U.S. president views the alliance itself not as a fixed strategic obligation but as a negotiable matter of cost.

Such an instance is not without precedent. It recalls President Trump’s June 2018 press conference in Singapore, following his first summit with Chairman Kim Jong Un, where he mentioned the possibility of reducing U.S. forces in Korea and halting the 'war games' symbolized by the combined exercises. Although President Trump asserted at the time that a withdrawal of U.S. forces was 'not part of the equation right now,' he made it clear that he ultimately wanted American soldiers to 'come home.' The move went beyond rhetoric. Later that year, the large-scale Ulchi Freedom Guardian and Key Resolve exercises were suspended, and additional drills were scaled back in 2019. What appeared in 2018 to be the idiosyncratic impulse of a particular president resurfaced in 2026 as a matter of official defense strategy. This suggests it may be a recurring structural feature of U.S. policy toward the Korean Peninsula whenever an administration led by Trump or one with a similarly transactional approach to alliance management is in power.

Taken together, these two cases raise a more fundamental question. Do South Korea, the United States, and Japan truly share a common threat perception regarding North Korea? Or has their security cooperation been far more conditional and asymmetrical from the outset than official joint statements imply? Beneath the veneer of alliance solidarity lie persistent differences in threat prioritization. These differences are shaped by three closely interconnected factors: (1) the variability of U.S. alliance policy, which treats the North Korean threat not as a fixed strategic priority but as one among many variables; (2) Japan’s gradual strategic pivot toward China, which relegates North Korea to a secondary threat; and (3) South Korea’s perception of the North Korean threat as an existential issue, a factor that can drive its security behavior in ways not fully shared by Washington or Tokyo. Understanding these differences is crucial as a diagnostic tool for predicting where ROK-U.S.-Japan trilateral cooperation is likely to fracture under future pressure.

The United States: From Commitment to Trilateral Cooperation to Burden-Shifting

The events of 2026 are best understood in the context of a broader U.S. policy shift foreshadowed by the 2026 National Defense Strategy (NDS) released earlier that year. In very direct terms, the NDS acknowledged that 'North Korea poses a direct military threat to the Republic of Korea as well as to Japan, both of which are U.S. treaty allies.' It also recognized that North Korea’s nuclear forces pose a 'clear and present danger of nuclear attack on the U.S. homeland.' This marked an explicit acknowledgment in America’s highest-level strategic planning document that the North Korean threat had evolved beyond a regional concern into an issue directly linked to the security of the U.S. homeland.

Despite this acknowledgment, however, the NDS concluded that 'South Korea can assume primary responsibility for deterring North Korea, with crucial but more limited U.S. support.' This reveals Washington’s intent to shift the burden of conventional deterrence to Seoul, with U.S. forces playing a more limited role. This apparent contradiction is resolved only when one recognizes that the NDS is not purely a threat assessment document but also a document about burden-sharing. The NDS is driven as much by fiscal constraints and a broader strategic pivot toward great power competition with China as it is by a reassessment of the North Korean threat itself. In effect, Washington appears to be selectively allocating its limited strategic resources, treating the 'conventional' dimension of North Korean deterrence as an area that can be delegated to Seoul, while reserving direct U.S. intervention for scenarios involving the homeland or higher-tier strategic competition.

This represents a departure from the direction set just three years prior. The 'Spirit of Camp David' joint statement of August 2023 committed the United States to 'conduct trilateral exercises on an annual, named, multi-domain basis on a regular basis.' This institutionalized what had been ad-hoc trilateral cooperation into a durable and explicit framework. At the time, President Biden declared that the trilateral relationship, 'built on the bedrock of our ironclad ROK-U.S. and U.S.-Japan alliances,' was 'stronger than it has ever been.' The three governments also jointly pledged to 'enhance strategic coordination between the U.S.-Japan and ROK-U.S. alliances and bring our trilateral security cooperation to new heights.' The rhetorical and institutional architecture of Camp David was designed to anchor trilateral cooperation as an enduring structural feature of the regional order, independent of the preferences of any particular administration.

The gap between this rhetoric and the burden-shifting logic of the 2026 NDS demonstrates that America’s commitment to deterrence is not a fixed structural feature of the alliance system but a variable that changes with the priorities of each administration. President Trump’s interventions in 2018 and 2026 have repeatedly demonstrated this unpredictability. What Camp David sought to institutionalize as permanent could be reversed within three years by a mere change in administration and presidential disposition.

Japan: A Threat Perception Centered on China

If the U.S. commitment is conditional, Japan’s threat perception is likely to differ from South Korea’s in a more persistent and structural manner. Although both Japan and South Korea are within range of North Korea’s missiles, the two countries do not perceive the threat in the same way. Nor does the North Korean threat occupy the same position in their respective hierarchies of strategic priorities.

Japan’s December 2022 strategy documents—the National Security Strategy, National Defense Strategy, and Defense Buildup Program—marked a significant turning point in its defense posture. These documents approved the acquisition of counterstrike capabilities and outlined a policy to double defense spending to 2% of GDP. The documents described North Korea’s military activities as a 'grave and imminent threat.' On the surface, this phrasing suggests continuity with South Korea’s threat perception. However, the same documents defined China as 'an unprecedented and the greatest strategic challenge,' placing it, not North Korea, at the center of Japan’s security response. The hierarchy implicit in this language is clear: North Korea is a grave and imminent threat, but China is an unprecedented and the greatest one.

This hierarchy became even more pronounced under Prime Minister Sanae Takaichi. Prime Minister Takaichi stated in the Diet that a Chinese attack on Taiwan could constitute a 'survival-threatening situation' for Japan, triggering a major diplomatic conflict with China. This incident illustrates how central the Taiwan-China axis has become in Japanese strategic thinking. Prime Minister Takaichi’s refusal to reaffirm Japan’s Three Non-Nuclear Principles and her stated plans to revise Japan’s three key security documents primarily in response to China’s military buildup further confirm that Tokyo’s strategic calculus is driven by China, not by developments on the Korean Peninsula. Public opinion data also supports this elite-level orientation. According to a survey by the University of Tokyo, 57.5% of Japanese respondents perceived China’s influence as expanding. This shows that a China-centric threat perception is not confined to security experts but has become mainstream.

Despite North Korea conducting six nuclear tests since 2006 and developing missile capabilities that can reach Japanese territory, there is little evidence that Japan views North Korea as an existential threat in the same way South Korea does. A poll by The Genron NPO illustrates the limited nature of this shift. Support for Japan’s nuclear armament in response to North Korea’s nuclear tests increased from 7% to 12.3% after the 2017 test. While this is a meaningful rise, it remains a minority view and is far lower than the level of anxiety among the South Korean public on the same issue.

This asymmetry in threat salience helps explain why Tokyo has remained hesitant to establish a trilateral extended deterrence framework with Washington and Seoul, despite years of encouragement from the United States since Camp David. Most tellingly, when the University of Tokyo survey asked which countries Japan should prioritize for security cooperation besides the United States, Japanese respondents ranked Taiwan (60.9%), Vietnam (40.3%), and Indonesia (42.5%) higher than South Korea (38.6%). This suggests that despite their shared status as U.S. allies and geographical proximity to North Korea, South Korea may not even be perceived as Japan’s most natural security partner. If the Japanese public does not instinctively choose South Korea as a security partner even on issues that could involve North Korea, the assumption of a natural trilateral alignment against Pyongyang needs significant revision.

This difference stands in stark contrast to the Cold War NATO model, where the Soviet Union was a clear, shared existential threat that could galvanize multilateral deterrence among geographically and politically diverse allies. NATO’s cohesion depended on the existence of a single, undeniable existential threat that overrode country-specific particularities. Today, no equivalent single threat binds Japan and South Korea together. North Korea is an existential threat to one country but merely a grave threat to the other. Conversely, China increasingly serves as the core organizing principle for Japan’s strategic thinking, but it does not play the same role for South Korea.

South Korea: North Korea as an Existential Threat

In contrast, South Korea’s threat perception is overwhelmingly North Korea-centric and existential in nature. North Korea’s massive artillery forces place the Seoul metropolitan area—home to roughly half of South Korea’s population and the overwhelming share of its economic output—at direct and constant risk. This is a vulnerability with no parallel in Japan’s security environment. North Korea’s repeated nuclear tests have transformed it from a hypothetical proliferation risk into a de facto nuclear-armed adversary. Since the early 2020s, global and regional uncertainty has been further heightened by Russia’s invasion of Ukraine and the deepening U.S.-China strategic competition. The Russian invasion demonstrated how vulnerable a non-nuclear state can be even with security commitments, while the intensifying U.S.-China competition has fueled concerns that American attention and resources could be diverted elsewhere. These factors have deepened both Seoul’s reliance on and its anxiety about U.S. extended deterrence.

South Korea’s approach to China, on the other hand, is relatively restrained and transactional rather than existential. According to a Chicago Council on Global Affairs survey, 55% of South Koreans believe China will become the country’s greatest threat within the next ten years. However, many South Koreans continue to see China as an essential actor in denuclearization efforts and for the long-term prospect of peaceful unification. This is a far more nuanced and less adversarial approach than Japan’s, which increasingly frames China in zero-sum terms as a strategic competitor.

This difference is also evident in attitudes toward a potential Taiwan Strait crisis. In 2021, the unprecedented phrase 'peace and stability in the Taiwan Strait' appeared in the joint statements of both the U.S.-Japan and the ROK-U.S. summits. This reflected growing U.S. expectations that both allies would speak more openly about stability in the Strait. However, Seoul has been far more hesitant than Tokyo to specify what role it might play in a Taiwan contingency. This is because South Korea’s strategic capacity is already consumed by the direct and existential threat posed by North Korea. Seoul’s restraint on the Taiwan issue does not signify indifference but reflects the fact that its limited strategic attention is already fully focused on Pyongyang.

Domestically, this existential perception creates recurring political pressures that Japan does not experience. The most significant driver of public support for nuclear armament in South Korea is the North Korean nuclear threat itself, compounded by growing doubts about the credibility of U.S. extended deterrence. Calls for the redeployment of U.S. tactical nuclear weapons or a NATO-style nuclear sharing arrangement often function more as devices to secure domestic persuasion than as carefully designed deterrence policies. South Koreans increasingly perceive the balance of power as shifting in Pyongyang’s favor. This reinforces Seoul’s persistent demands for stronger U.S. security commitments and will likely lead to an examination of the political viability of South Korea’s future nuclear options.

Divergent Threat Perceptions and Regional Instability

Synthesizing these three trends, the United States, Japan, and South Korea are not simply 'on the same side' when it comes to North Korea. Each actor assesses the threat differently for different reasons, with different consequences for regional stability.

The United States treats North Korea not as a fixed strategic priority but as one variable among many. This produces significant policy volatility between administrations and even within the term of a single administration. Japan subordinates the North Korean threat to its broader and increasingly dominant concerns about China, treating Pyongyang as a serious but ultimately secondary security problem compared to Beijing. Only South Korea views North Korea as an existential and constant threat, a perception that shapes nearly all of its security behavior, from public opinion on nuclear armament to its cautious stance on Taiwan.

This asymmetry has significant implications for regional stability. Uncertainty about U.S. alliance commitments is growing again, amplified by President Trump’s skepticism toward alliances and his repeated history of scaling back combined exercises in 2018 and 2026. This is likely to be felt most acutely in South Korea, which has a very high dependence on U.S. extended deterrence and relatively limited strategic alternatives. Japan, with its shifting threat perceptions and domestic political constraints under a leader like Takaichi, may also become more receptive to alternative or supplementary security cooperation. This is particularly likely in its relations with Taiwan and Southeast Asian partners, which could further diminish the centrality of ROK-U.S.-Japan trilateral cooperation in Tokyo’s strategic calculus.

In the short term, both Seoul and Tokyo are likely to seek pragmatic compromises with the United States. U.S. extended deterrence remains central to their defense postures, and neither has immediately viable alternatives. However, if perceptions of U.S. credibility continue to erode, compounded by the underlying differences in threat perception between Japan and South Korea, both countries may seek more supplementary and autonomous means of deterrence in the long run.

For South Korea, this could manifest as strengthened political momentum toward acquiring nuclear latency or independent nuclear armament. For Japan, it could lead to the further strengthening of counterstrike capabilities aimed primarily at China, not North Korea. Either path would likely weaken the coherence of trilateral security cooperation at a time when a unified response to North Korea is most needed. It would also likely complicate, rather than simplify, Washington’s strategic competition with China.

This analysis offers several policy implications for South Korea’s future strategic direction. First, South Korea must continue to maintain and strengthen its deterrence capabilities against North Korea with the United States. Even as the transition of wartime operational control (OPCON) and the realignment of U.S. Forces Korea (USFK) proceed, the alliance must be managed with the ROK-U.S. Mutual Defense Treaty and the combined defense posture as its core foundation. Furthermore, efforts to institutionalize and strengthen extended deterrence through the Washington Declaration, the Nuclear Consultative Group (NCG), and the Extended Deterrence Strategy and Consultation Group (EDSCG) must continue. This will be essential to preserve the credibility of deterrence even as changes in alliance burden-sharing and negotiations over defense costs unfold. Second, measures must be taken to prevent accidental military clashes near the Demilitarized Zone (DMZ). With North Korea defining inter-Korean relations as those between 'two hostile states,' even minor conflicts could escalate rapidly. Given Pyongyang’s current stance, priority should be given to identifying and mitigating factors of military and political conflict in the border area. The immediate top priority in managing inter-Korean relations should be the prevention of accidental crises. Third, peaceful coexistence should be pursued as a long-term framework for building a peace regime. While peaceful coexistence does not mean denuclearization or conflict resolution, it can provide a practical interim step for managing tensions, maintaining channels of communication, and preventing escalation. This approach could partially address North Korea’s concerns about the situation on the Korean Peninsula. However, peaceful coexistence should be presented not as a final state but as a transitional process toward a peace regime, ultimately aiming for peaceful unification. Simultaneously, efforts must be made to gradually reduce the root causes of the North Korean threat. ■

■ Hwang Jihwan_Professor, University of Seoul.

■ Edited by: Jaehyen Lim_Research Fellow, EAI; In-Hwan Oh_Senior Research Fellow, EAI

For inquiries: 02 2277 1683 (ext. 209) | jhim@eai.or.kr