| I. Framing the Issue: The Evolution of North Korea's Threats and Operationalizing Deterrence Cooperation

North Korea's nuclear and missile threats are no longer confined to a regional menace aimed directly at South Korea alone. By simultaneously pursuing short-range tactical nuclear delivery systems, solid-fuel intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and ballistic missile submarines (SSBNs), North Korea is integrating the Korean Peninsula, the Japanese archipelago, and the American homeland into a single deterrence equation. At the heart of this transformation is not a mere military buildup but an 'alliance decoupling' strategy designed to structurally erode the credibility of the U.S. extended deterrence commitment. As North Korea advances its capability to strike the American homeland, questions will inevitably grow as to whether the United States would genuinely risk nuclear strikes on Los Angeles or Washington in order to defend Seoul in the event of a crisis on the Korean Peninsula.

Against this shifting strategic environment, the three nations formalized a framework for trilateral cooperation through the Camp David Summit of August 2023 and the Trilateral Security Cooperation Framework (TSCF) of July 2024. Yet current ROK-US-Japan security cooperation remains largely confined to a lower tier of deterrence collaboration—centered on real-time missile warning data sharing, joint threat assessments, combined exercises, and the coordination of deterrence messaging toward North Korea. While these represent meaningful progress, they are insufficient given the pace at which North Korea's nuclear and missile capabilities are becoming increasingly complex across underwater, solid-fuel, and tactical-nuclear dimensions.

What this paper advocates is not a simple expansion of declaratory deterrence messaging or missile warning data sharing. What is needed going forward is the phased institutionalization of crisis-management and operational-support-oriented deterrence cooperation capable of addressing North Korea's underwater nuclear threats, solid-fuel ICBMs, and the possibility of tactical nuclear employment.

In light of the political sensitivities surrounding historical memory within South Korea, the political sensitivity toward Japan Self-Defense Forces (JSDF) activities, the command structure of the ROK-U.S. Combined Forces Command (CFC) and the United Nations Command (UNC), Japan's legal and institutional constraints, and the challenges of escalation management vis-à-vis China, ROK-US-Japan cooperation must evolve in a functional and phased manner rather than through treaty-alliance formation or wartime integrated operations. Accordingly, this paper aims to diagnose the gaps that the intensification of North Korea's nuclear and missile threats poses for the existing deterrence architecture centered on the ROK-US alliance, and to chart a realistic, step-by-step path—feasible within current political and legal constraints—for the development of intelligence sharing, joint threat assessment, anti-submarine detection and tracking, rear-base protection, and crisis management cooperation.

To this end, the paper proceeds as follows: first, it analyzes the changes in North Korea's nuclear and missile capabilities and their strategic implications; second, it examines the asymmetric impacts of these threats on South Korea, Japan, and the United States; third, it assesses the current state and constraining factors of ROK-US-Japan security cooperation; and fourth, it proposes a realistic, phased policy roadmap for deterrence cooperation.

| II. Changes in North Korea's Nuclear and Missile Capabilities and Their Strategic Implications

1. Expansion of Fissile Material Production and the Growing Nuclear Warhead Stockpile

One of the most significant changes in North Korea's nuclear program is the expansion of fissile material production capacity. The International Atomic Energy Agency (IAEA), in its 2025 report, simultaneously flagged the continued operation of the Yongbyon enrichment facility, increased cooling capacity, expanded use of the Kangson enrichment facility, the restart of the 5MW reactor at Yongbyon, and signs of reprocessing activity at the Radiochemical Laboratory. Former IAEA Deputy Director General Olli Heinonen warned in March 2026 that the restart of the Yongbyon light water reactor could expand North Korea's plutonium production capacity by as much as threefold 1).

Estimates of North Korea's nuclear warhead stockpile vary considerably by institution. The Stockholm International Peace Research Institute (SIPRI), in its 2025 Yearbook, assessed that North Korea had assembled approximately 50 nuclear warheads and possessed sufficient fissile material to produce up to 40 additional ones 2). By contrast, Lee Sang-gyu, Director of the Nuclear Security Research Division at the Korea Institute for Defense Analyses (KIDA), estimated that as of July 2025 North Korea possessed up to 150 nuclear weapons, a figure that could rise to 243 by 2030 and as many as 429 by 2040 3).

These divergences stem from differences in methodology. SIPRI's figures are closer to a lower bound, based on already-assembled warheads and relatively conservative assessments of fissile material stocks; KIDA's estimates are closer to an upper bound, converting North Korea's existing and producible quantities of plutonium and highly enriched uranium (HEU) into potential warhead numbers. The policy implications, however, are the same: North Korea is no longer content to hold a small arsenal as a political bargaining chip—it is systematically constructing a 'nuclear force' capable of being mounted on a diverse range of delivery vehicles.

2. The Multi-Layering of Missile Forces and the Lowering of the Nuclear Threshold

The qualitative transformation of North Korea's missile forces is proceeding simultaneously in both the short-range/tactical nuclear and long-range/strategic nuclear domains. In the short-range domain, the KN-23, KN-24, and 600mm large-caliber multiple rocket launchers are placing South Korea's Patriot and THAAD intercept systems under pressure through maneuvering trajectories and high mobility. Because these weapons have short flight times and compress the available window for detection, tracking, decision, and intercept, they impose a heavy burden on South Korea's existing missile defense architecture and crisis decision-making structure.

In the long-range domain, the emergence of solid-fuel ICBMs is the pivotal variable. The Hwasong-18, which North Korea successfully test-launched in 2023, recorded a maximum apogee altitude of 6,518 km, a range of 1,002 km, and a flight time of 74 minutes; Japan's Ministry of Defense assessed that on a standard trajectory its range could exceed 15,000 km. Solid-fuel ICBMs require shorter launch preparation times and offer greater concealability and mobility than liquid-fuel missiles, compressing the detection-decision-intercept cycle for the ROK, U.S., and Japan. The subsequent appearance of the Hwasong-19 in 2024 further advanced North Korea's long-range nuclear delivery capabilities.

A more serious concern is the lowering of the nuclear threshold. Through its Nuclear Forces Policy Law of 2022, North Korea stipulated that nuclear weapons may be used when it is judged that a nuclear or non-nuclear attack by hostile forces against the national leadership and the state's nuclear forces command authority 'has been launched or is imminent' 4). This means that even conventional precision strikes could be interpreted by North Korea as justification for nuclear use. As a result, ROK-U.S. discussions of preemptive strikes or decapitation operations simultaneously heighten deterrence and raise the risk of nuclear use in a crisis, functioning as a destabilizing factor.

3. The Nuclearization of Naval Forces and the Pursuit of Second-Strike Capability

The single most consequential variable transforming the long-term structure of the North Korean threat is the pursuit of a sea-based nuclear capability. North Korea launched the tactical nuclear attack submarine Hero Kim Kun Ok in September 2023 and has since continued to demonstrate SLBM and submarine-based cruise missile capabilities. CSIS Beyond Parallel assessed as of July 2025 that the vessel had not yet reached full operational status, but North Korea's political and military prioritization of an underwater nuclear strike capability is unmistakable 5).

Should North Korea actually acquire a survivable SLBM force, the ROK-US-Japan deterrence architecture would face a fundamental transformation. Unlike land-mobile launchers, submarines are difficult to locate at the point of launch, and preemptive neutralization in a crisis becomes virtually impossible. If North Korea were to acquire nuclear-powered submarines or a submarine force capable of extended underwater endurance, the existing kill-chain-centered response concept would confront inherent limitations. This would raise the cost of executing the U.S. extended deterrence commitment, simultaneously amplify security anxieties in South Korea and Japan, and substantially increase the need for trilateral maritime surveillance and anti-submarine warfare (ASW) cooperation among the three countries.

| III. Asymmetric Impacts on South Korea, Japan, and the United States

1. South Korea: Extremely Short Warning Times and Increased Risk of Nuclear Use

The greatest challenge confronting South Korea stems from the extremely short warning times created by geographic proximity. If North Korea's short-range ballistic missiles or large-caliber multiple rocket launchers were aimed at the Seoul metropolitan area or key military installations, South Korea would be required to detect the launch, assess the threat, issue intercept orders, alert the civilian population, and reach command-authority decisions within an extraordinarily compressed timeframe. In the case of short-range weapons systems that could potentially carry tactical nuclear warheads, distinguishing a conventional from a nuclear attack in the early moments of a crisis is exceptionally difficult.

Under these conditions, South Korea's deterrence policy faces two dilemmas. First, the more South Korea reinforces its preemptive strike capability, the more North Korea's leadership is likely to perceive this as a threat to regime survival and lower its nuclear use threshold accordingly. Second, the more South Korea depends on the U.S. extended deterrence commitment, the greater the likelihood that North Korea will attempt to decouple the alliance by threatening the American homeland. South Korea must therefore simultaneously reinforce the nuclear-based deterrence of the ROK-U.S. alliance while building autonomous response capabilities and crisis management capacity to address the possibility of low-intensity, opportunistic provocations and limited warfare.

2. Japan: Vulnerability of Rear Bases and the Burden of Escalation Management

The North Korean threat facing Japan operates on two levels. One is the direct strike threat against the Japanese mainland and U.S. military bases in Japan; the other is the strategic vulnerability arising from Japan's role as the key hub for U.S. reinforcement and rear-area support in the event of a Korean Peninsula contingency. If the U.S. bases at Yokosuka, Sasebo, Kadena, and Misawa were rendered non-functional by North Korean ballistic or cruise missile strikes, U.S. reinforcement operations for the Korean Peninsula could suffer severe delays or be effectively paralyzed.

Japan revised its National Security Strategy in 2022 to officially adopt a counterstrike capability and is pursuing increased defense spending. However, the expansion of Japan's military role is intricately linked to Korean Peninsula crisis management. If Japan's exercise of counterstrike capability is not sufficiently coordinated with South Korea's military operations or crisis management strategy toward North Korea, it could provoke North Korea's escalation calculus or generate political backlash within South Korea. Accordingly, Japan's role in ROK-US-Japan cooperation should be concretized primarily as: protection of U.S. bases in Japan, rear-area support, protection of sea lines of communication, missile warning data sharing, and ASW detection and tracking cooperation.

The issue of JSDF entry onto the Korean Peninsula is a matter of high political sensitivity within South Korea. If expanding ROK-Japan security cooperation is perceived as implying an automatic JSDF entry onto the Korean Peninsula, domestic opposition to the cooperation as a whole could intensify. It is therefore important to clearly establish the principle of South Korea's prior consent, and to separately frame rear-area support cooperation and operational participation within South Korean territory.

However, the possibility of JSDF direct entry onto the Korean Peninsula in defense of South Korea is in any case extremely slim, precisely because of North Korea's nuclear and missile threat against Japan. North Korea has claimed thermonuclear warhead delivery capability since its sixth nuclear test in 2017, and Japan's Ministry of Defense has assessed that North Korea has achieved the miniaturization and weaponization of nuclear warheads for delivery by ballistic missiles capable of reaching Japan, and that the Nodong and Scud-ER can serve as nuclear strike delivery vehicles against Japan 6). In essence, North Korea possesses thermonuclear-class capabilities and medium- and quasi-medium-range ballistic missiles capable of inflicting catastrophic damage on major Japanese cities and U.S. bases in Japan. Under these circumstances, JSDF direct combat engagement on the Korean Peninsula would expose the Japanese mainland and U.S. bases in Japan to the risk of North Korean nuclear and missile retaliation. Given Japan's experience of atomic bombings and its strong domestic anti-nuclear sentiment, the likelihood that Japanese society would support ground combat intervention on the Korean Peninsula—accepting such risks in order to defend South Korea—is exceedingly low.

3. The United States: Structural Pressure on Extended Deterrence Credibility

The United States' strategic dilemma is deepening in tandem with North Korea's ICBM capability development. The more North Korea advances its ability to threaten the American homeland with nuclear weapons, the more South Korea and Japan will inevitably question whether the United States would actually implement its extended deterrence commitment in the event of a Korean Peninsula contingency. This is the structural problem of extended deterrence commonly expressed as the 'Seoul-Los Angeles exchange equation'.

At the 56th Security Consultative Meeting (SCM) in 2024, the United States elevated the ROK-U.S. alliance to a 'nuclear-based alliance' and formalized nuclear deterrence and nuclear operations guidelines 7). This represents an important step forward, but documented commitments alone do not automatically guarantee the credibility of extended deterrence. Credibility can only be reinforced when operational interoperability is in place encompassing crisis-time nuclear consultation procedures, conventional-nuclear integrated planning, strategic asset operations, missile defense, and ASW detection and tracking.

| IV. The Current State of ROK-US-Japan Deterrence Cooperation and Its Constraining Factors

1. Distinguishing Deterrence/Diplomatic Cooperation from Wartime Operational Cooperation

The first distinction that must be drawn in any discussion of ROK-US-Japan security cooperation is between deterrence and diplomatic cooperation on one hand, and wartime operational cooperation on the other. Current ROK-US-Japan cooperation centers on: real-time sharing of North Korean missile warning data, joint assessments of the North Korean missile threat, combined exercises, coordination of deterrence messaging toward North Korea, and diplomatic coordination. This cooperation carries relatively high political acceptability and involves comparatively limited legal and institutional burdens.

By contrast, areas such as wartime operational plans, common operational pictures, integrated underwater operations, mutual access agreements, and a ROK-Japan military alliance entail far more complex political, legal, and operational constraints. ROK-US-Japan deterrence cooperation should therefore be pursued not by aiming immediately for wartime integrated operations, but by starting from intelligence sharing and crisis management and progressively accumulating limited, functional interoperability. This approach does not diminish the necessity of ROK-US-Japan cooperation: on the contrary, it is the condition that makes cooperation sustainable within the domestic and international political realities.

2. The Constraints of Domestic Politics and the Historical Issue

ROK-Japan security cooperation is not free from the constraints of South Korean domestic politics and historical memory, notwithstanding the intensification of the North Korean threat. In particular, the scope of JSDF activities, Japan's role in a Korean Peninsula contingency, military logistics cooperation, and the expansion of intelligence sharing are issues that can readily become politicized in South Korean society. If the imperative of security cooperation is emphasized while ignoring these constraints, the domestic foundation for cooperation could be undermined instead.

ROK-Japan security cooperation must therefore be pursued by managing historical issues and security cooperation separately, without either ignoring or circumventing historical issues. This principle should be established not as a long-term objective but as a fundamental and immediate starting point for cooperation. It is necessary to institutionalize the principle that, even when historical disputes arise, the minimum security communication channels for addressing the North Korean nuclear and missile threat will be maintained. At the same time, it must be continuously communicated that the purpose of cooperation is not to advance Japan's military expansion, but to protect the lives and safety of the South Korean people, to ensure the stable operation of U.S. reinforcement forces, and to prevent North Korean miscalculation.

3. Command Structure and Legal Constraints

The central pillar of military operations in a Korean Peninsula contingency is the ROK-U.S. alliance and the ROK-U.S. Combined Forces Command. The UNC plays an important role in the operation of rear bases in Japan, but the UNC's role and authority, the relationship among the Commander of United States Forces Korea, the Commander of the UNC, and the Commander of the ROK-U.S. Combined Forces Command, and the question of wartime operational control transfer all involve complex legal and institutional issues. ROK-US-Japan cooperation must therefore be designed not to replace or circumvent this command structure, but to complement the existing ROK-U.S. alliance architecture and the UNC rear support structure.

The question of JSDF entry onto the Korean Peninsula must also be approached within this context. The expansion of ROK-Japan cooperation does not mean automatic JSDF entry onto the Korean Peninsula, and the principle that JSDF military activities within South Korean territory require the explicit prior consent of the South Korean government must be clearly established. This is not a constraint on ROK-Japan security cooperation; it can instead serve as a political safety mechanism that enhances the domestic legitimacy of cooperation.

From a legal standpoint, the possibility of JSDF direct entry onto the Korean Peninsula is also limited. Article 9 of the Constitution of Japan renounces war and the threat or use of force as means of settling international disputes, and does not recognize the maintenance of war potential or the right of belligerency of the state 8). While the 2015 security legislation opened a limited possibility for the exercise of collective self-defense, the Japanese government continues to state that there is no change in the basic policy of being a peace-loving state and maintaining an exclusively defense-oriented posture. Japan's realistic role in a Korean Peninsula contingency is therefore likely to be limited to: homeland defense, protection of U.S./UNC rear bases in Japan, support for U.S. reinforcement forces, and missile warning data sharing not direct combat on Korean territory.

4. The Linkage Between a Taiwan Contingency and a Korean Peninsula Contingency

Should a U.S.-China military conflict occur in the Taiwan Strait, Kim Jong-un might conclude that a window of opportunity had opened to neutralize the Northern Limit Line (NLL) in the West Sea and seize the five West Sea islands. The United States' insufficient capacity to simultaneously respond at high intensity to both a Taiwan contingency and a Korean Peninsula contingency has been noted by numerous experts, including Under Secretary of Defense for Policy Elbridge Colby. While it cannot be assumed that China would actively support North Korean military provocations in the event of a U.S.-China military conflict, the possibility that China might acquiesce in or seek to make limited use of opportunistic North Korean provocations rather than actively suppressing them—should a U.S.-China military conflict become protracted and high-intensity—cannot be excluded. If the Ukraine war had not ended, Russia too might quietly welcome North Korean provocations as a means of diverting U.S. attention to the Korean Peninsula and Northeast Asia. North Korea could therefore attempt to seize portions of South Korean territory with the acquiescence and support of China and Russia.

The possibility of a linkage between a Taiwan contingency and a Korean Peninsula contingency is a core crisis management agenda item that the three countries of ROK, U.S., and Japan must examine together over the long term. Even now, confidential deliberations among the three countries on contingency scenarios in which a Taiwan contingency and a Korean Peninsula contingency erupt simultaneously must begin. Waiting until a crisis has begun to discuss response measures would already be too late.

However, examining the possibility of linkage between a Taiwan contingency and a Korean Peninsula contingency does not mean that South Korea should automatically be involved in Taiwan defense operations. South Korea's core interest lies in deterring North Korea from miscalculating a U.S.-China conflict as an opportunity for Korean Peninsula provocation, and in closing gaps in the Korean Peninsula defense posture through the stable operation of U.S./UNC rear bases in Japan.

At present, the appropriate approach to this issue is a layered separation. First, at the ROK-U.S. alliance level, military readiness posture and punitive messaging should be refined to deter opportunistic North Korean provocations. Second, at the ROK-U.S.-Japan level, intelligence sharing and joint assessment covering missile warnings, maritime surveillance, cyber threats, and North Korean military movements should be strengthened. Third, at the Japan level, consultations on the protection of U.S. bases in Japan and UNC rear bases should be conducted. Fourth, at the escalation management level vis-à-vis China, diplomatic communication channels should be maintained to prevent the crises on the Korean Peninsula and in the Taiwan Strait from mutually reinforcing one another.

| V. Policy Recommendations: A Phased Roadmap for ROK-US-Japan Deterrence Cooperation

ROK-US-Japan deterrence cooperation should be pursued by accumulating achievable functional cooperation, rather than aiming for 'military alliance formation' or 'wartime integrated operations'. The key lies not in the form of a treaty alliance but in the phased accumulation of predictable cooperation procedures and limited mutual support capabilities in times of crisis. A near-term, mid-term, and long-term roadmap for this purpose can be set out as follows.

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1. Institutionalization of Missile Warning Data Sharing and Joint Threat Assessment

The most realistic and urgent task is the institutionalization of North Korean missile warning data sharing and joint threat assessment. The three countries already operate a system for real-time missile warning data sharing, but there is a need to expand the scope of cooperation beyond the simple transmission of information to threat assessment and response message coordination. For example, if North Korea conducts a test launch of a short-range tactical nuclear delivery vehicle, a solid-fuel ICBM, or an SLBM and the three countries each issue different assessments and messages, North Korea will seek to exploit the gaps. Joint threat assessment is the most realistic instrument for enhancing deterrence without excessive institutionalization of ROK-US-Japan military cooperation.

2. ASW Detection and Tracking Cooperation in Response to the DPRK SLBM Threat

North Korea's underwater nuclear strike capability has not yet reached completion, but once operationalized, countermeasures become extremely difficult. The three countries must therefore strengthen ASW detection and tracking cooperation before the North Korean SLBM threat is fully realized. This cooperation should, however, be pursued in a limited and defensive form rather than as 'ROK-US-Japan integrated underwater operations', specifically: regularizing ASW exercises, standardizing detection and tracking procedures, examining the feasibility and scope of underwater acoustic data sharing, and sharing experience in operating maritime patrol aircraft.

South Korea has maritime surveillance capabilities in the East Sea and the Yellow Sea; Japan in the East Sea, the Korea Strait, the Tsugaru Strait, and the Soya Strait; and the United States across the broader Indo-Pacific. The three countries' capabilities are mutually complementary. Rather than binding these into integrated operations, cooperation should be designed to reduce gaps in situational awareness regarding North Korean submarine activities.

3. Rear Base Protection in Japan and UNC Rear Support Cooperation

Japan's strategic significance in a Korean Peninsula contingency lies in the stable operation of U.S. bases in Japan and the UNC rear bases. North Korea, in the event of a war on the Korean Peninsula, is likely to seek to delay U.S. reinforcement operations by striking military installations in South Korea as well as U.S. bases and rear support hubs in Japan. The priority of ROK-US-Japan cooperation should therefore be placed on: protection of U.S. and UNC rear bases in Japan, logistical support, protection of sea lines of communication, and ensuring the stable movement of U.S. reinforcement forces.

In this regard, a ROK-Japan Acquisition and Cross-Servicing Agreement (ACSA) is worth examining as a mid-term objective. In the near term, the first step should be to assess the necessity, scope, domestic legal issues, and prior consent procedures of an ACSA, with a view to pursuing limited negotiations or formalized crisis logistics procedures in the mid-term.

An ACSA should not be targeted for immediate conclusion but should be pursued incrementally, following sufficient deliberation on its necessity, scope of application, prior consent procedures, and operational modalities in contingencies—taking domestic public opinion and legal issues into account.

4. Confidential Crisis Management Consultations on Taiwan Contingency-Korean Peninsula Linkage

The possibility of a simultaneous Taiwan contingency and Korean Peninsula contingency is a sensitive matter difficult to address publicly, yet a strategic risk that the three countries of ROK, U.S., and Japan cannot afford to ignore. A situation in which the United States must simultaneously respond at high intensity to military crises in the Taiwan Strait and on the Korean Peninsula could provide North Korea with incentives for opportunistic provocations. The three countries should therefore examine simultaneous-crisis scenarios within a confidential crisis management consultative body, rather than announcing a publicly integrated strategy.

The core of these consultations consists of four elements: first, intelligence sharing to enable early identification of signs of opportunistic North Korean provocation; second, the ROK-U.S. alliance's immediate response posture in the event of North Korean provocation; third, protection of U.S. and UNC rear bases in Japan; and fourth, escalation management vis-à-vis China and maintenance of diplomatic communication channels. This approach acknowledges the necessity of ROK-US-Japan cooperation while mitigating domestic concerns in South Korea that the country might be automatically entangled in a Taiwan contingency. In other words, the purpose of consultations related to a Taiwan contingency is not South Korea's automatic participation in Taiwan defense operations, but deterrence and crisis management preparation to prevent a Taiwan Strait crisis from cascading into a Korean Peninsula provocation.

5. The Question of JSDF Entry onto the Korean Peninsula and the Principle of ROK Prior Consent

To ensure the sustainability of ROK-Japan security cooperation, the question of JSDF entry onto the Korean Peninsula must be addressed clearly rather than avoided. South Korean societal concern does not stem from security cooperation with Japan per se, but from distrust as to whether JSDF forces could operate on the Korean Peninsula in a crisis without South Korea's consent. The South Korean government must therefore make clear that the expansion of ROK-Japan security cooperation does not imply automatic JSDF entry onto the Korean Peninsula, and that JSDF activities within South Korean territory require the explicit prior consent of the South Korean government.

This principle is not a constraint on ROK-Japan cooperation but a political safety valve that makes cooperation possible and a key condition for securing domestic acceptability. The clearer the prior consent principle is, the less domestic opposition there will be in South Korea, and Japan would also be able to clearly understand the legal and political boundaries of such cooperation. It is therefore necessary to examine this matter confidentially within future ROK-Japan 2+2 security dialogues or high-level ROK-US-Japan consultations, while domestically pursuing a strategic communication approach that separately frames rear-area support cooperation and operational participation within South Korean territory.

Domestically, it is also necessary to explain that concerns about 'automatic JSDF entry onto the Korean Peninsula' are exaggerated. Without an alliance relationship between South Korea and Japan, JSDF forces entering South Korean territory to defend South Korea would require the explicit request and consent of the South Korean government as a prerequisite. However, Japan lies within the range of North Korea's nuclear warheads and ballistic missiles such as the Nodong and Scud-ER, and North Korea could cite JSDF intervention in a crisis as justification for nuclear and missile strikes against U.S. bases in Japan and the Japanese mainland. It is therefore realistically improbable that the Japanese government would dispatch JSDF forces to ground combat on the Korean Peninsula in defense of South Korea accepting the risk of North Korean nuclear retaliation, in the face of Japan's experience of atomic bombings and domestic anti-nuclear sentiment.

6. Strategic Communication for Securing Domestic Acceptability

One of the greatest challenges for ROK-US-Japan deterrence cooperation is domestic acceptability. No matter how serious the North Korean nuclear and missile threat may be, policy cannot be sustained unless the public understands both the necessity and the limits of cooperation. The government and the expert community must therefore communicate the purposes of ROK-US-Japan cooperation in the following terms:

ROK-US-Japan cooperation is not about supporting Japan's military expansion, but about protecting the lives and safety of the South Korean people from North Korea's nuclear and missile threats.

ROK-US-Japan cooperation does not replace the ROK-U.S. alliance; it supplements the deterrence capability of the ROK-U.S. alliance.

ROK-Japan cooperation does not imply automatic JSDF entry onto the Korean Peninsula and must be premised on the principle of South Korea's prior consent.

Consultations related to a Taiwan contingency are not about South Korea's automatic entanglement, but about crisis management preparedness to deter opportunistic North Korean provocation.

Given that North Korea could target Japan with nuclear and missile attacks, the likelihood of JSDF direct combat intervention on the Korean Peninsula in defense of South Korea is slim: the practical focus of ROK-Japan security cooperation lies in rear base protection and support for U.S. reinforcement forces.

Ultimately, the policy value of ROK-US-Japan deterrence cooperation lies in narrowing the deterrence gaps created by North Korea's nuclear and missile threats in a realistic and phased manner. North Korea seeks to separately pressure South Korea, Japan, and the United States. In response, South Korea must maintain the ROK-U.S. alliance as its central axis while progressively strengthening intelligence sharing, ASW cooperation, rear base protection, and crisis management cooperation with Japan and the United States. This is the most realistic path for ROK-US-Japan deterrence cooperation under current political constraints.

In the long term, if ROK-US-Japan security cooperation accumulates sufficient trust and institutional foundations, the conditions may form for the ROK-Japan relationship to develop into a more structured security partnership that transcends current functional cooperation. This is not a policy goal for the present moment, but a strategic possibility to be kept open in response to long-term changes in the security environment on the Korean Peninsula.

1. Cho Sang-jin, "North Korea Estimated to Have Up to 50 Nuclear Weapons... Light Water Reactor Restart Could Triple Production Capacity," Voice of America (Korean Service), March 13, 2026.
2. Stockholm International Peace Research Institute (SIPRI), "Nuclear risks grow as new arms race looms-new SIPRI Yearbook out now," June 16, 2025.
   https://www.sipri.org/media/press-release/2025/nuclear-risks-grow-new-arms-race-looms-new-sipri-yearbook-out-now.
3. Lee Sang-gyu, "Analysis of Recent Changes in North Korea's Nuclear Weapons Production Capability and Denuclearization Considerations," KIDA Security Strategy FOCUS, July 17, 2025.
4. "Decree of the Supreme People's Assembly of the Democratic People's Republic of Korea on the Nuclear Forces Policy of the Democratic People's Republic of Korea," Rodong Sinmun, September 9, 2022.
5. Joseph S. Bermudez Jr., Victor Cha, and Jennifer Jun, "North Korea's First Ballistic Missile Submarine Still Not Operational," CSIS Beyond Parallel, July 17, 2025.
6. 38 North, "Sixth Nuclear Test Detected at Punggye-ri, Declared to be a Hydrogen Bomb," September 3, 2017; Japan Ministry of Defense, "Recent Missile & Nuclear Development of North Korea (Oct 2025)," Security Environment Surrounding Japan, November 21, 2025.
7. U.S. Department of Defense, "56th Security Consultative Meeting Joint Communiqué," October 30, 2024.
   https://www.defense.gov/News/Releases/Release/Article/3951794/56th-security-consultative-meeting-joint-communique/.
8. Article 9, Constitution of Japan; Government of Japan, Japan's Legislation for Peace and Security, March 2016.

※ The opinions expressed in Sejong Focus are those of the author and do not represent the official views of the Sejong Institute.