The Future of Extended Deterrence with South Korea: Implications for the Region and Beyond

The October 2025 Summit between Presidents Trump and Lee Jae Myung in Gyeongju marked a turning point in the U.S.-ROK alliance and the emergence of increased nuclear cooperation as a foundational element of alliance stability. The United States endorsed South Korea’s development of nuclear-powered attack submarines, committed to supporting ROK civil uranium enrichment and spent fuel reprocessing, and reaffirmed extended deterrence backed by the full range of U.S. capabilities, including nuclear. Seoul committed to increased defense spending and $25 billion in U.S. military equipment purchases by 2030, while accepting the predominant role in the conventional defense of the peninsula. Whether this announcement provides a new foundation for an enduring extended deterrence relationship or a waypoint to a fundamentally different regional nuclear future remains an open question.

The Evolving Nuclear Deterrence Landscape

Driven by a deteriorating security environment and evolving alliance dynamics, the credibility of U.S. extended deterrence to the Republic of Korea has come under growing pressure over the last five years, resulting in a persistent assurance deficit among the ROK public and political classes.

These deterrence anxieties are rooted in shifts in the security environment, both on the Peninsula as well as across the region.  Expanding nuclear capabilities across the Indo-Pacific, burgeoning strategic partnering between DPRK and Russia, and concerns about the adequacy of the U.S. nuclear posture to meet the full scope of deterrence requirements in the region have placed growing strain on the U.S.-ROK extended deterrence relationship. The DPRK’s expanding and  increasingly survivable nuclear arsenal, coupled with increasingly reliable long-range delivery systems, prompts concerns that the DPRK nuclear threat to the U.S. homeland could undermine U.S.-backed extended deterrence in a regional conflict.

China’s rapidly expanding nuclear arsenal and suite of delivery systems places growing demands on U.S. deterrence and feeds perceptions that the U.S. deterrent could be “spread too thinly” to credibly deter China, Russia, and the DPRK from regional aggression. Russia’s ability to wage conventional war “under the nuclear shadow” in Ukraine has also heightened concerns that the United States may fail to respond forcefully to regional aggression for fear of nuclear escalation. Also growing security cooperation and alignment between DPRK and China, and more recently between DPRK and Russia, raises concerns that one or the other will backstop regional adversaries in the face of U.S. deterrence threats. 

Deterrence anxiety, however, is not just a function of external factors, but also reflects shifting confidence in the nature and reliability of the alliance itself. For most of the last seventy-five years, the U.S. forces stationed on ROK territory have served as the visible foundation of this assurance relationship—the essential glue binding both countries in a shared “community of fate.”  Significant strains in the extended deterrence relationship had already emerged in the Biden Administration, leading to a range of initiatives to bolster confidence in the extended deterrence relationship. These included creation of the Nuclear Consultative Group to engage in more formalized consultation as a step towards modest shared decision-making, conducting more visible visits by U.S. bomber aircraft and nuclear-armed submarines, adoption of new capabilities including the nuclear-variant sea-launched cruise missile (SLCM-N), and expanding exercises and engagements especially in the area of conventional nuclear integration.

It was clear, however, in the early days of the Trump administration that “shared” stakes would be a tenuous basis for stabilizing nuclear deterrence in the region, especially given clear pressures to reduce and reorient U.S. forces resident on the peninsula. The October 2025 Gyeongju summit and its aftermath represented a significant acceleration of this effort, emphasizing that South Korea would play “a leading role in conventional defense of the Korean Peninsula,” Subsequently, the January 2026 National Defense Strategy formalized this division of labor from the U.S. side, stating plainly that “South Korea is capable of taking primary responsibility for deterring North Korea with critical but more limited U.S. support.” It seems that the conventional-nuclear division of labor in the alliance is being actively redrawn. Sustaining a viable extended deterrence relationship will require that U.S. assurances assume more tangible and visible forms, as well as the substitution of alternative forms of shared interest and destiny, or risk a broader dismantling of the bilateral alliance.

Alternative Futures for the U.S.-ROK Nuclear Relationship

Recent events suggest that a repositioning and recalibration of the U.S.-ROK extended deterrence relationship is underway. What is less clear is whether these steps mark the beginning of a more transformational shift in the nature of the alliance and the role of nuclear weapons within it.  With this uncertain future in mind, this paper offers the following four alternative futures, predicated primarily on different visions for the nuclear dimension of the U.S.-ROK relationship, although each carries significant implications for the broader alliance and for the regional security environment in the Indo-Pacific.

Future 1: They Go Alone—ROK Independent Nuclear Deterrence

A decision to formally decouple ROK nuclear deterrence from our own would be transformational across the region and trigger a broader fundamental questioning among other allies, acutely Japan and Australia, but in Europe as well. Barring the restoration of a negotiated peninsula-wide commitment to denuclearization, it is difficult to imagine such a decoupling would not compel the ROK to actively develop an independent nuclear capability, or at least the ability to develop one on an expedited timeline.

Over the last decade, calls for an independent ROK nuclear deterrent have become both more frequent and more mainstream, with a significant percentage of the ROK population signalling support for nuclear independence. Under the leadership of the prior President, Yoon Suk Yeol, calls to either strengthen extended deterrence or consider an independent nuclear option intensified,  though it remains unclear how ROK domestic opinion would shift when faced with the actual financial and geopolitical costs of such a move. The ROK would have to build significant covert capabilities without discovery and almost certainly violate the safeguards agreements on existing nuclear capabilities. Withdrawal from the NPT would be inevitable if not immediate. The only state to ever withdraw from the Nuclear Nonproliferation Treaty is North Korea (2003), cementing its position as a pariah state excluded from the international system and its benefits. For the ROK and its robust export-based economy, economic sanctions would be inevitable and profoundly disruptive, even if temporary. The likely permanent banning of the ROK from the nuclear power exports business would cost more than $20B annually, and the country’s domestic nuclear energy program, (responsible for nearly one-third of ROK electricity requirements), would suffer major disruption, and significantly jeopardize U.S.-ROK nuclear energy cooperation. Despite the many obstacles, and current South Korean President Lee Jae Myung’s strong preference for U.S. nuclear deterrence over a South Korean nuclear capability, such a “They Go Alone” future is not impossible, especially in the wake of a collapse of the NPT architecture or radical restructuring of the security frameworks in the Pacific, and in the term of a future South Korean President in 2030 or after.

In an “almost” version of this future, the United States could decide to actively cooperate with the ROK to reduce its fuel cycle dependencies by supporting independent ROK enrichment and reprocessing capabilities and encouraging the accumulation of nuclear fuel stocks in the country. Such steps would allow the ROK to more closely approach the nuclear threshold while allowing both the United States and ROK to remain compliant with their international legal obligations. The October 2025 Gyeongju summit, in which the United States formally endorsed ROK civil uranium enrichment and spent fuel reprocessing and approved construction of ROK nuclear-powered attack submarines, suggests that such a future is plausible. However, such capabilities fall far short of the fully capable, deliverable, and reliable nuclear arsenal that would be of deterrent value vis-à-vis the DPRK, and enrichment and reprocessing facilities would pose tempting preemptive targets for North Korea without the benefit of an independent counterstrike option. Furthermore, the submarine approval, while significant, stops well short of a weapons capability. The steps taken at Gyeongju may, over time however, lower the threshold for more ambitious nuclearization, particularly under a future ROK president less committed to the alliance framework and the ROK’s nonproliferation leadership. Any ROK transition from nonnuclear ally to credible nuclear possessor would be costly and dangerous, with profound regional implications. 

Future 2: The New Old Deal—Revamped Nuclear Commitments with a Restructured Partnership

In this largely transactional future, the United States would remain generally committed to its nuclear obligations for the defense of South Korea while substantially reducing the numbers of U.S. military personnel on the peninsula, and sustaining its extended nuclear deterrence commitments to the ROK through a restructured set of arrangements. In important respects, the October 2025 Gyeongju summit and the January 2026 NDS together represent a partial realization of this future already underway.  At the same time, Congress has stepped in to restrict substantial reductions in the U.S. military presence on the Peninsula without robust consultation.

Beyond what has already been announced, nuclear-relevant elements of this future could include increasing regional, rotational deployments of U.S. nuclear assets—particularly air and maritime—to include rotational dual-capable aircraft, port visits of nuclear-capable naval assets, and visible commitment of new SLCM-N capability to the defense of the peninsula. Expanded large-scale exercises with additional conventional/nuclear integration components could help demonstrate nuclear resolve. Clear declaratory statements reinforcing U.S. commitment to extended nuclear deterrence—and public and private assurances that key U.S. capabilities will remain fenced for a DPRK scenario even in the face of simultaneous crisis or conflict with China—would address one of Seoul’s most persistent anxieties. Additional  tangible, visible demonstrations of nuclear cooperation that carry assurance weight independent of ground force numbers could reinforce confidence in extended deterrence.  Conventional military cooperation, favorable FMF/FMS terms to support highly advanced, independent ROK military capabilities expanding co-production facilities for high value munitions and delivery systems, and relaxing military technology transfer restrictions for advanced U.S. military technology such as latest generation missile defense systems might be seen as compensatory for reduced permanent force presence.

Success in this future would also be enhanced through the issuance of a new economic and security pact linking favorable terms, new access privileges, and assurances of fenced nuclear capabilities in crisis or conflict that could be portrayed as modernizing the relationship for current needs and requirements to the benefit of both countries.  Even so, this approach will likely falter along the lines of previous “stop-gap” approaches, either as a short-term “deal” to buy time until the ROK can achieve an independent nuclear capability or a more enduring relationship can be renegotiated. Finally, Japan and possibly Australia could reasonably expect equivalent treatment on any expanded nuclear cooperation terms.

Future 3: We Go (Nuclear) Together—A Nuclear Deterrence-Centered Partnership

In this future, the United States and ROK would significantly expand nuclear cooperation and integration, emphasizing nuclear deterrence as the principal means of preventing DPRK aggression and stabilizing the region in the face of China’s growing nuclear capabilities. The ROK would formally and fully assume independent responsibility for the conventional defense of the peninsula, while the U.S. would limit its support to intelligence and warning, missile defense, maritime security, and above all, nuclear deterrence. The U.S. would provide significantly enhanced control, visibility, and availability of U.S. nuclear capabilities while also expanding missile defense commitments to the peninsula through some combination of the following:

• Regular rotational deployment of dual-capable F-35 aircraft, preferably including the ROK acquisition of the most advanced, nuclear-capable variant, with training of ROK pilots for the DCA mission and potential certification of ROK nuclear-capable aircraft for nuclear weapons delivery.
• Repurposing and renovation of facilities for temporary, rotational, or crisis deployment of nuclear weapons, including construction and certification of required weapons storage facilities and the infrastructure necessary to support temporary forward-deployment of U.S. nuclear forces.
• Substantial expansion of sea-based non-strategic nuclear capabilities like the SLCM-N publicly committed to the defense of Korea.
• Deepening of ROK involvement in nuclear planning and operations, including permanently assigned ROK personnel to USSTRATCOM headquarters and a USSTRATCOM planning element at ROKSTRATCOM headquarters.
• Integration of ROK into Golden Dome development and acquisition, and permanent basing of U.S. theater nuclear capabilities in Guam to anchor regional deterrence against both the DPRK and China.
• Expansion of U.S. overall non-strategic nuclear capabilities in terms of naval and air platforms and associated limited-yield nuclear weapons, including rapid expansion of the production pipeline for the B-61-12.

This future could also be consistent with a far more complex and controversial proposal: the establishment of a formal nuclear sharing arrangement granting ROK armed forces nuclear control and employment responsibilities for U.S. nuclear weapons during wartime following launch authorization by the U.S. President.

The permanent physical presence of U.S. nuclear weapons carries significant assurance weight in some alliance quarters. However, the cost and risk downsides of such an approach are profound. Permanently stationed nuclear weapons would require extremely expensive construction and sustainment of weapons storage and delivery infrastructure. The additional numbers of B-61 weapons needed would unduly strain the NNSA and weapons lab production complex to the detriment of other higher priority modernization efforts. And permanently stationed weapons would be well inside the target ranges of multiple Chinese and DPRK nuclear and conventional missile systems, potentially encouraging preemptive counterforce targeting rather than deterring it.

As for an official nuclear sharing arrangement, unlike the impression conveyed by some analysts, nuclear sharing is not intended to convey choice or decision authority for nuclear employment to the ally—only the operational control to execute decisions once made. Practical implementation would almost certainly depend upon permanent basing of U.S. weapons in the ROK and formal certification of ROK forces to store, move, load, and deliver U.S. weapons via their own dual-capable aircraft. The magnitude of creating and maintaining this status while ensuring state-of-the-art security and surety cannot be underestimated.  Finally, Chinese negative reactions to any perceived nuclear “ramp up” would also put substantial pressure on the alliance with possible knock-on effects for U.S. China policy.

Future 4: We Go All-Together—A Multilateral Pacific Defense Pact

In this future, the United States and ROK would agree to fundamentally transform their bilateral defense commitments into a broader multilateral pact—with the inclusion of states like the Philippines, Australia, and Japan. Such a “Pacific Defense Pact” would bind member nations into a collective commitment to mutual defense in the face of any adversary, be it the DPRK, China, or another would-be attacker.

From an extended deterrence perspective, a pact comprised of states that already enjoy the benefit of the U.S. nuclear umbrella would not significantly expand U.S. deterrence requirements since the bilateral commitments already exist. If the nuclear umbrella aspect of mutual defense pact extended beyond current allies—for instance to the Philippines—then careful consideration should be given to if, how, and when the current nuclear umbrella should be widened. Nuclear-relevant features of such a pact could include deployment to Guam of U.S. theater nuclear capabilities and associated permanent or rotational dual-capable aircraft, strengthening regional deterrence writ large rather than versus the DPRK specifically. Incorporating defense pact countries into a broader missile defense architecture would improve the ability to defend the U.S. homeland and better defend allies. Longstanding South Korean and Japanese desires to participate in nuclear sharing arrangements more like those utilized in NATO could be addressed collectively and more equitably than through sequential bilateral negotiations. Formal planning, declaratory policies, and exercises would ensure sufficient capability is retained to deter DPRK aggression even if faced with other contingencies. Japan’s emerging “one theater” concept—framing the Korean Peninsula, East China Sea, and surrounding areas as a single operational space—and the trilateral U.S.-Japan-ROK pledge signed in New York in September 2025, which reaffirmed extended deterrence commitments to both allies and committed to deeper crisis consultation mechanisms, point in a similar direction.

Expanding rotational nuclear commitments under such a pact, especially if additional countries are added to the U.S. nuclear umbrella, will strain an already overstretched U.S. nuclear weapons modernization program. Attempting such an approach could also alienate South Korea, fail to achieve its ambitions, and generate resistance to more politically realistic options. Despite a small but growing body of support in Seoul and Washington for such a multilateral pact, neither of the two main political parties has warmed to the prospect, and ROK political elites remain deeply wary of the entrapment risks and near-certain Chinese opposition that would accompany it. 

Conclusion

Each of the above futures offers a vision for sustaining vital U.S. interests in the Indo-Pacific and deterring North Korean military adventurism should the United States wish to reduce or eliminate its permanently stationed forces in the ROK. Each is predicated on different expectations for the role of nuclear deterrence, its viability in the face of shifting geopolitical dynamics, and the commitments both the United States and the Republic of Korea would need to make to ensure a credible and capable deterrence posture.

What makes these choices more urgent—and more consequential—is that they will not be made in isolation. The nuclear decisions the United States and the Republic of Korea reach over the coming years will reverberate across the Indo-Pacific in ways that are neither predictable nor fully controllable. Japan is watching closely. Tokyo’s own extended deterrence relationship with Washington runs through many of the same pressure points—China’s buildup to a nuclear peer to the United States, doubts about U.S. will, and growing domestic debate about the adequacy of Japan’s non-nuclear posture in a region where North Korea has operational nuclear weapons. A visible weakening of the U.S.-ROK nuclear relationship would put pressure on the U.S.-Japan relationship in ways that are difficult to contain bilaterally. Australia has embarked on its own path of nuclear adjacency through AUKUS—nuclear-powered submarines under conventional armament, rotational U.S. bomber and submarine presence, and deepening integration with U.S. strategic strike planning. Seoul now appears to be on a similar track, and the question of whether these developments reinforce each other within a coherent regional deterrence architecture, or evolve as parallel and potentially competing frameworks, will itself be a powerful driver shaping U.S. nuclear strategy decisions in the Indo-Pacific for the foreseeable future.