The Possibility of Strategic Divergence between South Korea and Japan in a Taiwan Contingency and South Korea's Response
Executive Summary
Although trilateral security cooperation among South Korea, the United States, and Japan has been institutionalized through the Camp David agreements, South Korea and Japan are highly likely to diverge in their level of response to a Taiwan contingency due to fundamental differences in the design of their respective alliances with the United States. Since the inauguration of the Takaichi administration, Japan has signaled that it could designate a Taiwan Strait contingency as an 'existential crisis situation,' showing readiness to actively participate in the US-led deterrence framework. In contrast, South Korea maintains strategic ambiguity, prioritizing deterrence against North Korea and the management of relations with China. This divergence is not a policy failure but a structural phenomenon stemming from the asymmetry of the Cold War-era hub-and-spokes alliance system. The South Korean government should approach this divergence as a condition to be managed rather than overcome, separating information sharing, rear-area support, and direct intervention into distinct phases, and establishing an upper limit for its participation in advance. At the same time, anchoring the rationale for trilateral cooperation back to North Korean deterrence, while coordinating with the United States on the scope of the US Forces Korea (USFK) role, wartime operational control (OPCON) transfer, and the UN Command (UNC) rear bases, is required as a short-term task.
I. Situation Analysis
The Possibility of Strategic Divergence between South Korea and Japan in a Taiwan Contingency: Situation Analysis
1. Background and Progress
Trilateral security cooperation among South Korea, the United States, and Japan entered an institutionalization phase starting with the Camp David Summit in August 2023. At the time, the leaders of the three countries adopted the 'Camp David Principles' and the 'Commitment to Consult' [6]. In August of the following year, as a practical implementation of these agreements, the trilateral joint exercise 'Freedom Edge' was conducted, and a memorandum of understanding on security cooperation was signed [6]. While this framework ostensibly began under the pretext of responding to North Korea's nuclear and missile threats, a shared strategic motivation to counter China lay beneath the surface [3].
The problem is that the foundation of this cooperation was asymmetrical from the very beginning. Due to long-standing historical disputes and mutually negative public sentiment, bilateral cooperation between South Korea and Japan struggled to develop organically and has generally progressed only within the framework of trilateral cooperation mediated by the United States [15]. This is a legacy of the 'hub-and-spokes' system established by the United States during the Cold War [15]. This explains why security cooperation between South Korea and Japan has been perceived as the 'weakest link' in trilateral cooperation [15].
A study published in a domestic academic journal in 2025, titled 'Alliance under Pressure: Japan, South Korea, and Strategic Divergence in a Taiwan Contingency,' analyzed this structural asymmetry by applying it to a Taiwan contingency scenario [2]. The study raised the possibility that Japan and South Korea could exhibit different response patterns within the US alliance system during a Taiwan contingency [2]. Its core argument is that while Japan has left open the possibility of active intervention based on its geographical proximity and the revision of its security legislation, South Korea operates under a separate strategic calculus of deterring North Korea and managing relations with China [2].
The background to this potential divergence lies in Japan's independent shift in security policy. Through the revision of its key security documents, Japan designated China as an explicit security competitor, pledged to increase its defense budget to 2% of GDP, and pursued the acquisition of counterstrike capabilities and the establishment of an integrated deterrence system [11]. This shift is evaluated as largely reflecting the National Security Strategy of the United States [11]. In contrast, South Korea has prioritized nuclear deterrence against North Korea through the Nuclear Consultative Group (NCG) and maintained strategic ambiguity regarding the Taiwan issue [17].
2. Current Situation
Since the inauguration of the Takaichi administration, Japan's stance has become even clearer. In November, Prime Minister Takaichi mentioned that a Chinese blockade of Taiwan could constitute an 'existential crisis situation' for Japan [14]. This statement opens up the legal basis for exercising the right to collective self-defense [14]. It is interpreted as signaling Japan's willingness to participate more actively in US-led deterrence efforts in the event of a Taiwan contingency [14].
This stance is translating into actual military measures. While conducting large-scale joint exercises starting October 19, the United States and Japan plan to deploy the US military's new Navy Marine Expeditionary Ship Interdiction System (NMESIS), a land-based anti-ship missile system, to Yonaguni Island, Japan's westernmost point, for the first time [7]. Chinese media warned that, given the geographical proximity to Taiwan, this deployment could reinforce the perception that Japan and the United States might intervene in the Taiwan issue, thereby increasing the risk of miscalculation [7]. This demonstrates that China views Japan's expanding involvement in Taiwan as a factor that heightens regional tensions [7].
In contrast, signals have emerged in South Korea-US relations indicating that the stability of the alliance commitment itself is faltering. In August 2026, President Trump ordered a reduction in South Korea-US joint military exercises, citing costs and his willingness to improve relations with North Korea [16]. Subsequent exercises were shortened in duration, reduced in scale, and concluded earlier than scheduled [16]. This is interpreted as a signal that the US president views the alliance not as a fixed strategic obligation but as a negotiable cost issue [16]. Consequently, signs of a rift among the three countries are appearing even in their perception of the North Korean threat, which had served as the common foundation for trilateral cooperation [16].
At the same time, observers point out that the war in Iran has exposed a gap between the United States' commitment to prioritize the Indo-Pacific and its actual allocation of resources [5]. As US strategic attention is redistributed back to the Middle East, concerns are rising within Japan that the role and burden expected of Japan in a Taiwan contingency could relatively increase [1][5].
3. Key Actors and Positions
The Japanese government is moving toward keeping the possibility of intervention in a Taiwan contingency open legally and politically under the Takaichi administration [14]. The mention of potentially designating the situation as an existential crisis symbolically demonstrates this shift in stance [14]. Accepting the deployment of missiles on Yonaguni Island also falls within this context [7]. However, the extent to which this proactive stance will be supported by domestic public opinion in Japan and the operational realities of the Self-Defense Forces remains an issue to be separately examined.
The South Korean government has maintained strategic ambiguity regarding the Taiwan issue and refrained from taking a clear stance between the United States and China [17]. The background to this cautious approach is the risk that South Korea's relations with China could deteriorate sharply if it accedes to US demands to intervene in Taiwan [17]. At the same time, while the Lee Jae-myung administration in South Korea has sent active signals toward strengthening cooperation with Japan [15], there remains a possibility that differences in positions between the two countries could be exposed on sensitive security issues such as the Taiwan question.
The United States seeks to continue promoting trilateral cooperation as a platform to counter China, but the Trump administration's transactional view of alliances raises questions about the consistency of its alliance commitments, as seen in the reduction of joint military exercises [16]. At the same time, the war in Iran has revealed the reality that it is difficult for US strategic assets to be concentrated solely in the Indo-Pacific [5].
China defines Japan's moves to expand its involvement in Taiwan as a security threat and is sending warning messages by raising the risk of miscalculation regarding specific military measures such as the missile deployment [7]. Even after the Trump-Xi summit, assessments suggest that uncertainty continues to persist in the Taiwan Strait, contrary to the surface-level calm [4][12].
4. Key Issues
The first issue is the effectiveness of trilateral minilateral cooperation among South Korea, the United States, and Japan. Although the Camp David framework has progressed at the level of joint statements and consultation mechanisms [6], the divergence of interests between South Korea and Japan could act as a limitation on cooperation in scenarios involving actual military risk-sharing, such as a Taiwan contingency [2]. The fundamental cause is an asymmetry: South Korea is concerned about the deterioration of relations with China and a security vacuum on the Korean Peninsula resulting from intervention in a Taiwan contingency, whereas Japan possesses unavoidable security interests due to its geographical proximity [2][17].
The second issue is the changing credibility of US alliance commitments. The reduction of South Korea-US joint military exercises [16] heightens questions about the extent to which South Korea can trust the US commitment to support it during a Taiwan contingency. Conversely, this serves as an incentive for Japan to strengthen its position within the alliance through independent defense build-ups and active contributions to the United States [11][14].
The third issue is the potential vacuum in the regional deterrence posture. With Japan's proactive stance and South Korea's cautious approach coexisting, China may have an incentive to exploit this divergence to conduct grey-zone provocations or psychological warfare [7]. The situation in which trilateral cooperation shows signs of cracking even on the North Korean issue [16] raises fundamental questions about whether trilateral coordination would function in a higher-intensity scenario like a Taiwan contingency.
II. In-Depth Analysis
The Possibility of Strategic Divergence between South Korea and Japan in a Taiwan Contingency: In-Depth Analysis
1. Root Cause Analysis
The strategic divergence between South Korea and Japan is not a new phenomenon. Its roots lie in the fundamentally different nature of the alliances each country has with the United States. From its inception, the US-Japan alliance was designed as a regional security mechanism with China and the Soviet Union in mind. In contrast, the South Korea-US alliance began as a bilateral defense treaty to counter threats from North Korea. This design difference continues to operate in the same manner even after more than 70 years.
For Japan, the Taiwan Strait is close to a matter of survival. The distance from Yonaguni Island to Taiwan is a mere 110 kilometers. Prime Minister Takaichi's remark that a Chinese blockade of Taiwan could be designated as an 'existential crisis situation' is the result of translating this geographical proximity into legal language [14]. For South Korea, however, the Taiwan Strait is geographically distant, and in terms of security priorities, it is secondary to North Korea's nuclear and missile threats. The South Korean government's concentration of resources and diplomatic efforts on the Nuclear Consultative Group (NCG) clearly reflects this priority.
A more fundamental cause is the difference in their economic exposure to China. South Korea has a high trade dependence on China and has experienced China's informal economic retaliation following the THAAD deployment [8]. This experience impressed the cost of managing relations with China upon both the South Korean government and businesses. While economic relations with China are also important for Japan, it has already shifted its direction toward prioritizing security considerations over economic ones by designating China as an explicit competitor in its revised security documents [11]. This asymmetry fundamentally differentiates the options available to the two countries in a Taiwan contingency.
2. Structural Context
Security Structure: Trilateral cooperation among South Korea, the United States, and Japan is a product of the hub-and-spokes system [15]. The South Korea-US alliance and the US-Japan alliance exist independently, and there is no official bilateral defense treaty between South Korea and Japan. Although the 'commitment to consult' among the three countries was documented in the Camp David agreements [6], this is not a legally binding mutual defense treaty. What role the US Forces Korea (USFK) would play and whether South Korea would provide rear-area support in a Taiwan contingency remain ambiguous. While this ambiguity is manageable in peacetime, it carries the structural risk of leading to coordination failures among allies in the event of a crisis.
Political Structure: Through the revision of its security documents, increases in defense spending, and the acquisition of counterstrike capabilities, Japan has accumulated a significant domestic political consensus on a hardline stance toward China [11]. On the foundation of this accumulated consensus, the Takaichi administration has secured the political ground to express a more explicit intent to intervene [14]. In contrast, South Korea's domestic politics have seen its policies toward Japan and China fluctuate significantly with changes in administration. Examples include the deterioration of South Korea-Japan relations and the stagnation of trilateral cooperation during the Moon Jae-in administration, followed by the rapid restoration of relations under the Yoon Suk-yeol administration [9]. Although the Lee Jae-myung administration has stated that it will inherit the stance of cooperation with Japan, skepticism remains as to whether this policy will be sustained throughout its five-year term [9]. Furthermore, the instance in August 2026 where President Trump ordered a reduction in South Korea-US joint military exercises demonstrates that the United States itself treats the alliance not as a fixed strategic obligation but as a negotiable cost [16]. This variable shakes South Korea's trust in the United States and could act to lower South Korea's incentive to participate in a Taiwan contingency.
Economic and Public Opinion Structure: Mutually negative sentiments among the publics of South Korea and Japan have long hindered the organic development of security cooperation [15][13]. Although the two countries sent messages of strengthening cooperation at the recent Lee Jae-myung–Ishiba summit [15], whether this will translate into public support during a high-intensity crisis like a Taiwan contingency is a separate matter. Within Japan, hardline public opinion toward China has consolidated to a certain extent since the revision of the security documents, whereas the South Korean public's perception of the Taiwan issue as directly linked to its own national security remains relatively low.
3. Historical Precedents and Similar Cases
The closest precedent is the reduction of South Korea-US joint military exercises during the 2018–2019 period. Immediately after the Singapore US-North Korea summit, President Trump mentioned the possibility of halting 'war games,' which actually led to the suspension of the Ulchi Freedom Guardian and Key Resolve exercises [16]. This decision, which was dismissed as a personal impulse at the time, recurred in 2026 at the level of official defense strategy [16]. This is a repeated example showing that US alliance commitments are variable depending on the disposition of the administration, suggesting that similar uncertainties could be projected onto the division of labor the United States might demand from South Korea and Japan in a Taiwan contingency.
Another reference case is the conflict between South Korea and China over the THAAD deployment [8]. This experience of paying an economic price to China when South Korea participated in US-led security measures continues to act as a persistent deterrent in the South Korean government's decision-making regarding Taiwan. Japan has had relatively fewer corresponding experiences, making its sensitivity to Chinese economic retaliation lower than that of South Korea.
Looking at the history of South Korea-Japan security cooperation itself, the real-time sharing of North Korean missile warning data agreed upon at the Phnom Penh trilateral summit in November 2022 is symbolic [13]. This demonstrates that military cooperation between South Korea and Japan could only progress when mediated by the common threat of North Korea. The implication of this precedent is that the same level of cooperation is difficult to replicate on issues where the threat perceptions of the two countries differ, such as the China-Taiwan question. The fact that the United States has previously refrained from pursuing trilateral missile defense cooperation out of consideration for South Korea's sensitivities and China's backlash [13] is proof that the United States itself was aware of the difference in temperature between South Korea and Japan regarding their policies toward China.
4. Key Variables Shaping Future Developments
First, the continuity of Japanese domestic politics. The key is whether the Takaichi administration's interpretation of an existential crisis situation will be maintained even after a change of government [14]. Although Japan's defense strategy shift has acquired a certain degree of bipartisan support, the actual invocation of the right to collective self-defense depends heavily on the personal political decision of the prime minister.
Second, changes in the US commitment to the Korean Peninsula. South Korea's calculus regarding participation in a Taiwan contingency will vary depending on whether the Trump administration's directive to reduce joint military exercises is a one-off measure or a structural shift [16]. As the United States strengthens its approach of treating alliances as a cost issue, South Korea's incentive to distance itself from the Taiwan issue based on its own independent threat assessment will grow.
Third, the intensity of China's grey-zone pressure. Regarding the deployment of NMESIS to Yonaguni, China has already warned of 'increased security risks if Tokyo intervenes in the Taiwan issue' [7]. If China pairs such military signaling with economic retaliation, South Korea is highly likely to take an even more cautious stance, recalling the THAAD precedent.
Fourth, whether the Lee Jae-myung administration's policy toward Japan will be sustained. Whether the message of strengthening cooperation at the summit [15] will lead to a substantive deepening of security cooperation, or whether it will fluctuate according to changes in the political situation as in the past, will serve as a touchstone for assessing the effectiveness of trilateral minilateral cooperation [9].
III. Final Recommended Responses
The Possibility of Strategic Divergence between South Korea and Japan in a Taiwan Contingency: Final Recommended Responses
1. Comprehensive Assessment and Recommended Responses
The strategic divergence between South Korea and Japan is not the result of a policy failure. It is a structural phenomenon stemming from different starting points in their alliance designs. The US-Japan alliance was designed with countering China and the Soviet Union in mind. The South Korea-US alliance began with the primary goal of deterring North Korea. This difference was not bridged even by the Camp David agreements [6]. If the South Korean government approaches this divergence as an 'obstacle to be overcome,' it is highly likely to repeat past policy failures.
Therefore, the basic direction should be to acknowledge the divergence while managing it. Attempts to steer trilateral cooperation into an automatic intervention mechanism in a Taiwan contingency would trigger domestic political backlash in South Korea and excessively raise the cost of managing relations with China [8][17]. Conversely, rejecting cooperation altogether would result in South Korea's isolation from Japan's increasingly proactive stance toward China under the Takaichi administration [14] and the United States' deterrence strategy against China [12]. The key is to formally design a 'differentiation of participation levels.' An approach is needed that separates information sharing, rear-area support, and direct military intervention into distinct phases, and clearly defines South Korea's upper limit of participation in advance.
At the same time, the rationale for trilateral cooperation must be re-anchored to North Korean deterrence. Reaffirming that the initial rationale for the Camp David framework was responding to North Korea's nuclear and missile threats is the way to preserve South Korea's leverage [3][6]. However, a key variable is that the perception of the North Korean threat among the three countries has recently been faltering. The instance where President Trump ordered a reduction in South Korea-US joint military exercises [16] shows that the US commitment to North Korea itself can be treated as a negotiable cost. Rather than leaving this rift unaddressed, South Korea must pursue parallel diplomatic efforts to place the North Korean deterrence agenda back at the center within the framework of trilateral cooperation.
2. Short-, Medium-, and Long-Term Action Plans
Short-Term (Within 6 Months)
The South Korean government should initiate prior coordination with the United States through closed-door channels regarding the scope of the role of U.S. Forces Korea (USFK) in a Taiwan contingency. It is necessary to clarify legal and administrative boundaries to ensure that the wartime operational control (OPCON) transition system and the issue of United Nations Command (UNC) rear bases do not automatically lead to involvement in a Taiwan contingency. This initiative should be jointly pursued by the Ministry of National Defense and the Ministry of Foreign Affairs.
While maintaining summit-level communication channels between the Lee Jae-myung administration and the Takaichi administration, South Korea should create an opportunity to publicly reaffirm its position of "strategic ambiguity" on the Taiwan issue [15][17]. The stance of reaffirming "unwavering South Korea-Japan and South Korea-U.S.-Japan cooperation" at the August South Korea-Japan summit [15] should be maintained, but managed carefully so that it is not interpreted as a commitment to intervene in Taiwan.
A monitoring system for U.S.-Japan military movements, including the deployment of NMESIS on Yonaguni Island, should be institutionalized on a permanent basis within the Ministry of National Defense [7]. This serves as a leading indicator for assessing regional tension levels.
Medium-Term (1–2 Years)
It is necessary to separate the agenda of South Korea-U.S.-Japan trilateral cooperation by function. Areas where South Korea can participate without political burden—such as sharing North Korean missile warning data, maritime security, and humanitarian assistance and disaster relief (HADR)—must be institutionally distinguished from politically sensitive areas, such as direct military cooperation in a Taiwan contingency. This is a way to maintain the effectiveness of the Camp David framework while preserving South Korea's room for maneuver [6].
At the bilateral level, South Korea and Japan should stabilize the existing stance of separating historical issues from security cooperation. The fact that a foundation of support for South Korea-Japan cooperation is forming at the public level, as shown by the South Korea-Japan Joint Public Opinion Poll [15], broadens the scope for policy implementation. However, this support must be approached with a clear understanding that it does not extend to the Taiwan issue.
In relations with China, efforts to reduce economic vulnerability should continue, drawing lessons from the THAAD dispute [8]. Diversifying supply chains and managing dependence on China are prerequisites for expanding South Korea's strategic options in a Taiwan contingency.
Long-Term (3 Years or More)
While raising the level of institutionalization of South Korea-U.S.-Japan cooperation beyond the Camp David agreement, its content should be designed in a way that does not infringe upon South Korea's strategic autonomy toward China and North Korea. This starts with distinguishing South Korea-U.S.-Japan cooperation from plurilateral frameworks in which South Korea does not participate, such as the Quad or AUKUS. Japan's move to pursue a "more autonomous and resilient partnership" with Australia [10] shows that Japan is seeking diversification away from a U.S.-centric approach. South Korea also needs to consider a similar diversification strategy in the long term.
At the same time, South Korea should flesh out its own Indo-Pacific strategy to make it clear that the "South Korean Indo-Pacific Strategy Framework" is not merely a rhetorical response to U.S. demands, but a strategy based on its own independent interests [3].
3. Monitoring Indicators and Trigger Points
Whether the Takaichi administration recognizes a "survival-threatening situation" and the progress of related legal procedures should be used as key indicators [14]. If this actually leads to the exercise of the right to collective self-defense, the United States is highly likely to request some form of role-sharing from South Korea as well.
The trend of expanding U.S.-Japan military deployments in the Nansei Islands, including Yonaguni Island, must be continuously tracked [7]. Whether China defines this as an "increasing security risk" and escalates its backlash serves as a leading signal for the next stage of tension escalation [7].
Changes in the Trump administration's policy stance toward China and Taiwan are also key variables. Following the Trump-Xi summit, assessments have emerged that the Taiwan issue remains in "limbo" [4]. The point at which U.S. policy toward Taiwan shifts from ambiguity to clarity will be a turning point that forces a choice on both South Korea and Japan [12].
Changes in the credibility of the U.S. alliance commitment itself must also be tracked. If signals of the United States treating alliances as a "negotiable cost" are repeated, as in the case of directing the reduction of joint ROK-U.S. military exercises [16], this could affect not only South Korea's but also Japan's trust in the United States. The "gap between U.S. Indo-Pacific priorities and actual policy" exposed by the Iran war [5] also needs to be watched in the same context.
Domestically, the trend of support for security cooperation in the South Korea-Japan Joint Public Opinion Poll must be continuously observed [15]. If the foundation of support weakens, the government's room for maneuver to expand cooperation will shrink.
4. Summary and Conclusion
South Korea-Japan strategic divergence is not something to be eliminated, but managed. This is a structural phenomenon stemming from the historical asymmetry of alliance design, and with the Takaichi administration strengthening its hardline stance toward China [14], the gap is highly likely to widen rather than narrow for the time being. South Korea's response should be to re-anchor the rationale for South Korea-U.S.-Japan cooperation to North Korean deterrence, while differentiating its level of participation in a Taiwan contingency by function. Through this, South Korea should secure room to control the costs of managing relations with China and domestic political burdens, without completely breaking away from U.S.-led deterrence efforts. At the same time, considering the new variable of the variability of U.S. alliance commitments [16], parallel efforts are needed to restore the foundation of trust in South Korea-U.S.-Japan cooperation itself.
References
[1] [The Diplomat] Iran War Highlights 2 Challenges for Japan in a Taiwan Contingency
[4] [Nikkei Asia] The Trump-Xi summit leaves Taiwan in limbo
[5] [Brookings Institution] The Iran war and America’s widening strategy gap in Asia
[10] [The Diplomat] Japan and Australia: Toward a More Autonomous and Resilient Partnership
[12] [Foreign Affairs] The Coming Taiwan Crisis
[14] [Foreign Affairs] Japan Is Giving the United States a Chance in Asia
*This text is an AI translation of an original written in Korean. Some translations or nuances may be inaccurate.
This report is an in-depth analysis planned by an EAI researcher, grounded in sophisticated AI-assisted research, and finalized by the EAI researcher.
How can I help you?