U.S.-ROK Alliance

For the past seven decades, the United States’ extended deterrence commitment to the Republic of Korea (ROK) has been critical to advancing two central aims of the alliance: (1) deterring North Korean aggression and (2) preventing nuclear proliferation by giving South Korea a credible alternative to developing its own nuclear arsenal. Today, however, a shifting geopolitical environment has introduced new areas of uncertainty into the alliance.

The Asan Institute for Policy Studies and CSIS engaged in a joint project from late 2025 to early 2026, including a full-day workshop to assess extended nuclear deterrence cooperation within the U.S.-ROK alliance. The select group of experts and former officials discussed the credibility of U.S. extended deterrence commitments and brainstormed ways to strengthen these commitments amid heightened uncertainty and evolving security challenges in the region.

The key observations and recommendations of the project recorded in this report do not necessarily reflect the views of every participant in the study, or the positions of either organization.

Build on Existing Institutional Capacity

• Further strengthen and institutionalize U.S.-ROK extended deterrence mechanisms, including the Nuclear Consultative Group (NCG) and Extended Deterrence Strategy and Consultation Group (EDSCG).

• Multilateralize strategic dialogues, initiatives, and exercises.

• Consider developing an overarching regional dialogue on extended deterrence that connects and coordinates all the regional U.S. extended deterrence mechanisms.

• Initiate a trilateral U.S.-Japan-ROK dialogue on extended deterrence.

• Consider adding Australia to some U.S.-Japan-ROK security discussions and initiatives.

• Consider initiating regional nuclear arms and missile limitation talks.

• Explore security commitments below the nuclear level with partners outside the neighborhood.

• Energize multilateral Track 1.5 dialogues on extended deterrence.

Maintain U.S. Forces on the Peninsula

• When considering alliance modernization efforts, including wartime operational control (OPCON) transition, Washington and Seoul should keep in mind the critical role that U.S. troops on the peninsula play in upholding credible extended deterrence.

  • The actual number of forces is less important than the degree to which U.S.-ROK combined capabilities, cooperation, and alignment remain steady.

Closely Coordinate and Manage Expectations Regarding the U.S.-ROK Nuclear-Powered Submarines (SSN) Deal

• Establish U.S. interagency, U.S.-ROK bilateral, and Track 2 processes to clarify details of the SSN deal.

Maintain Regular Deployments of Strategic Assets to the Peninsula

• The key is for the deployments to be regularized and rotational to serve as an important reminder of U.S. capabilities rather than discrete deployments in response to a specific event.

• The nuclear-armed sea-launched cruise missile (SLCM-N) can provide a powerful deterrent that is less vulnerable to preemptive attacks than forward-deployed U.S. tactical nuclear weapons on the peninsula (see below).

Consider Pre-Decisional Discussions on U.S. Tactical Nuclear Weapons Redeployment and Nuclear Sharing

• The redeployment of U.S. tactical nuclear weapons to the Korean Peninsula could involve several advantages, including presenting a physical, visible deterrent, but there are also considerable operational and strategic downsides, including vulnerability and cost.

• The introduction of a nuclear sharing mechanism would be another form of deterrence, but would also involve significant complications.

_Seek Crisis Management and Tension-Reduction Mechanisms with the Democratic People’s Republic of Korea (DPRK) Without Weakening the Alliance or Deterrence Capabilities

• The United States and South Korea should establish a forum to discuss and coordinate North Korea’s policy positions with each other before any meetings with Pyongyang.

• Negotiations with the DPRK should maintain denuclearization as a stated aim but be realistic about its low prospects.

• Efforts to reduce tensions with the DPRK should avoid steps that would affect deterrence, such as suspending U.S.-ROK military exercises.

Devise a Public Communications Strategy to Increase Knowledge About Nuclear Issues

• Form a joint nuclear options study group to increase public awareness of the pros and cons of various nuclear strategies.

• Devise initiatives to increase nuclear literacy among the public without instilling panic and fear that create new assurance dilemmas.

• Consider sharing U.S. space-based early warning data with South Korea and Japan following North Korean missile tests to ensure coordinated assessments among the three allies.

Introduction

A Pivotal Moment for Extended Deterrence

The U.S. extended deterrence commitment to the ROK is grounded in the U.S.-ROK Mutual Defense Treaty of 1953. It entails a pledge by the United States to dedicate the full range of its military capabilities, including nuclear, to deter and, if needed, defeat an external attack against South Korea. For the past seven decades, the pledge has played a critical role in pursuing two central aims of the U.S.-ROK alliance: (1) deterring North Korean aggression and (2) preventing nuclear proliferation by giving South Korea an alternative to developing its own nuclear weapons.

Recent high-level meetings and bilateral statements have affirmed the value that both the U.S. and South Korea continue to place on the alliance as the linchpin for peace, security, and prosperity on the Korean Peninsula and in the region. Nonetheless, workshop participants identified four developments that have created new challenges for the U.S. capacity and will to deter North Korean aggression.

First, North Korea has further developed its nuclear and missile capabilities. Following DPRK leader Kim Jong-un’s unusually comprehensive and precise announcement of the goals of his weapons development in January 2021, Pyongyang fast-tracked its efforts to produce three legs of a strategic nuclear triad, including intercontinental ballistic missiles, submarine-launched ballistic missiles (SLBMs), and maneuverable or multiple reentry vehicles, as well as tactical nuclear delivery systems. As one participant noted, this rapid progression in capabilities—combined with Kim’s declaration of a nuclear first-use policy in 2022 and codification of nuclear weapons possession in the country’s constitution in 2023—removed prior skepticism among South Koreans that North Korea might use its nuclear arsenal for more than bargaining leverage to pursue coercive or revisionist goals.

Second, cooperation between North Korea, China, and Russia has deepened, further strengthening Pyongyang’s capabilities. Shortly before Russian President Vladimir Putin invaded Ukraine in February 2022, China and Russia declared a partnership with “no limits.” This spurred a number of actions by Beijing and Moscow to destabilize the U.S.-led international order that directly benefited North Korea, such as using veto power in the UN Security Council to block punitive actions against Pyongyang and undermining existing sanctions against North Korea. Kim, in turn, began to directly support Putin’s efforts in the Ukraine war, providing ammunition, ballistic missiles, and troops. This allowed North Korea to improve its conventional military capabilities and give its soldiers real battlefield experience and familiarity with cutting-edge technologies including drone warfare. North Korea’s involvement in the war has also incentivized Russia to provide food, fuel, and technological support to Kim’s weapons development drive, albeit in unconfirmed areas.

Third, this tightening authoritarian alignment has occurred against a backdrop of intensifying U.S.-China strategic competition and broader changes in the regional security environment, with direct implications for South Korea. As one workshop participant noted, China’s nuclear expansion—involving a purported goal to attain 1,500 nuclear warheads by 2035—has prompted Seoul to pay closer attention to Beijing’s capabilities as a source of concern, not just for the United States but for South Korea down the road as well. The Taiwan issue has also become a more direct concern for Seoul, as the United States and South Korea have begun to consider the possibility that contingencies on the Korean Peninsula and in the Taiwan Strait could become interconnected. The global erosion of norms against nuclear first use, spurred by Russian and North Korean statements, has added to the complex of challenges in the regional security environment.

Lastly, domestic political shifts in Washington and Seoul have injected new areas of uncertainty into the U.S.-ROK alliance. The first year of the second Trump administration has involved conflicting dynamics. On one hand, U.S. President Donald Trump has made many significant efforts to upgrade deterrence architecture and enhance South Korea’s status as a strategic maritime and technological partner for the United States, including through his support for South Korea’s pursuit of SSN and deepened bilateral cooperation in the shipbuilding industry. Equally significant is what has not changed: U.S. troop levels on the peninsula are still around 28,500, U.S.-ROK military exercises continue, and extended deterrence commitments remain intact. A fact sheet released, albeit belatedly, following the largely successful August 25 and October 29 meetings between President Trump and South Korean President Lee Jae Myung in Washington and Gyeongju, respectively, highlighted a range of new and enduring areas of security and economic cooperation.

On the other hand, President Trump’s transactional style and frequent linkage of economic and security issues have eroded trust in the U.S.-ROK relationship. Three events in particular have created unease in Seoul regarding Washington’s commitment to the alliance. First, U.S. hesitation to robustly support Ukraine in its war efforts against Russia, including under the administration of President Joe Biden, has made South Koreans wonder whether Washington would come to Seoul’s aid in the event of a North Korean attack. Second, trade and tariff negotiations that disregard U.S. treaty commitments under the Korea-U.S. Free Trade Agreement (KORUS FTA) have called into question the degree to which the Trump administration might also consider undermining U.S. commitments under the 1953 U.S.-ROK Mutual Defense Treaty. Debates over burden sharing and troop deployments now seem, in the words of one participant, more “structural features of U.S. domestic politics” than episodic disputes. Third, and most visibly, the September 2025 U.S. Immigration and Customs Enforcement (ICE) raid of a Hyundai-LG battery plant construction site in Georgia, which involved hundreds of Korean nationals being placed in handcuffs and shackles due to suspected visa violations, left psychological scars in the minds of everyday South Koreans.

Collectively, these dynamics demonstrate that the U.S. role in the world is changing in fundamental ways as Washington takes deliberate steps away from the rules-based order that it helped build in the years following World War II, even as authoritarian powers such as China and Russia seek to challenge the United States. These shifts create some positive opportunities for South Korea to increase its autonomy in ways that the Trump administration supports under the banner of greater burden sharing in the alliance. But they have also yielded new questions regarding the enduring nature of the U.S. commitment to the alliance.

Seoul has weathered some dramatic political changes over the past year. South Korean President Lee took the reins of government in June 2025 following months of chaos sparked by former President Yoon Suk Yeol’s failed martial law bid and impeachment. Many suspected that Lee, hailing from the left side of the ROK’s political spectrum as a member of the Democratic Party of Korea, would clash with President Trump, as prior progressive South Korean leaders have with Republicans in office in the United States, but this did not materialize in 2025. Trump and Lee are well aligned in supporting engagement with North Korea and greater autonomy for South Korea in providing for its own defense. Overall, President Lee has prioritized maintaining smooth relations with Washington, as indicated by the first two summit meetings with Trump in August and October 2025. But a contentious turn in U.S.-ROK trade or security negotiations, or further incidents like the Georgia ICE raid, could jeopardize Lee’s ongoing capacity to buffer the U.S.-ROK alliance from domestic criticisms and pressure from hardcore progressive elements.

These observations suggest new risks in the complacency about extended deterrence that prevailed in the United States for around 50 years—a period when, as one participant put it, the U.S. approach was “essentially on automatic.” The inflection point for extended deterrence was identified as 2022–23, when old vehicles were recognized as insufficient to meet new challenges. The Biden and Yoon administrations took marked steps to improve this situation by issuing the landmark Washington Declaration of April 2023 and establishing the NCG thereafter, which introduced new structures and processes to strengthen extended deterrence. The Trump and Lee administrations have, so far, upheld the principles of the Washington Declaration and supported the work of the NCG. But domestic pressures, alongside growing regional and global challenges, could pull these leaders in different directions. As the alliance navigates this evolving, perilous landscape, Americans and South Koreans must grasp the high stakes of extended deterrence–related decisions in the near future.

Returning to First Principles

Why Extended Deterrence?

Given the shifts underway on the Korean Peninsula and in the world, it is important to consider the following question: Does extended deterrence remain a viable and effective way for the United States and South Korea to pursue shared interests in managing the North Korean threat while maintaining regional stability? Thoroughly addressing this question requires identifying the spectrum of possible approaches available to the United States and South Korea to pursue these objectives, identifying the pros and cons of each, and determining the best path forward based on that analysis (see Table 1).

First, the United States and South Korea could invade North Korea, find its nuclear weapons, and destroy them. This might resolve the issue of nuclear weapons (if all of them could be located) but would entail a severe risk of triggering a war.

Second, the United States and South Korea could attempt to denuclearize North Korea through negotiations. This approach is consistent with long-standing U.S. and South Korean policies, and there are many reasons why maintaining denuclearization as a stated aim remains a good idea. Abandoning the aim of denuclearization would convey that the United States no longer prioritizes upholding the Non-Proliferation Treaty (NPT) regime it helped create during the Cold War to prevent the global spread of nuclear weapons. Countries such as South Korea and Japan, which may consider developing such programs for their own defensive purposes, would presumably see both lower costs and new incentives for doing so and behave accordingly.

The downside of denuclearization through negotiations is that it appears out of step with facts on the ground. Given the mature status of North Korea’s program and geopolitical shifts in North Korea’s favor, denuclearization would entail a long pathway and is highly unrealistic in the near term. Decades of attempting to convince North Korea to give up its nuclear weapons through various negotiation modalities have not been successful. More recent efforts to open channels of dialogue under the Biden and second Trump administrations have similarly failed. Since the brief meeting between Kim and Trump in Panmunjom in June 2019, North Korea has shown no interest in engaging with the United States.

▲ Table 1: Possible Approaches to Managing the North Korean Threat

Even a less-ambitious negotiating strategy focused on limiting, rather than eliminating, North Korea’s nuclear program is unlikely to decrease the North Korean threat in a meaningful way. For instance, if the United States and South Korea were to pursue an interim cap-and-freeze agreement to slow or halt North Korea’s production of nuclear weapons, Kim would likely give up obsolete systems he no longer needs or slow down fissile material production—a process that could easily be reversed.

Negotiations with North Korea could have important implications for crisis management, as well as some modest positive effects on military confidence building at the conventional level. But expectations for limiting or eliminating North Korea’s nuclear program through negotiations need to be adjusted to reality: Neither approach is likely to be effective in stopping Kim from retaining a nuclear force capable of posing an acute threat to the U.S.-ROK alliance.

Third, South Korea could develop its own nuclear weapons. President Trump has often stressed that South Korea is a very wealthy, capable state. During his 2016 campaign, he even went so far as to say in an interview with the New York Times that the idea of South Korea and Japan building their own arsenal is “a position that at some point . . . we have to talk about,” as “if the United States keeps on its path . . . they’re going to want to have that anyway.”

But a South Korean indigenous program would have several downsides for both South Korea and the United States. For South Korea, it is dangerous. The country would be extremely vulnerable to North Korean preventive attacks during the estimated three-to-five-year window it would take for South Korea to develop nuclear weapons (and up to 10 years for a survivable capability). China and Russia might also get involved—if not through engaging in preventive attacks directly, then through assisting North Korea in doing so. The possibility of the U.S. commitment to South Korea waning over this period would make the situation even graver for Seoul.

Even if South Korea were to avoid preventive attacks and achieve a nuclear capability, its possession of nuclear weapons would invite new security dilemmas and escalatory risks. As participants noted, “As soon as you [South Korea] have them, you’re on China’s target list, you’re on Russia’s target list. That’s unavoidable.” Missile flight times in the event of an attack on South Korea from North Korea would be around seven or eight minutes (versus around 24 minutes for cross-Pacific trajectories from North Korea to the United States). Developing force protection strategies for these scenarios would likely trigger new levels of North Korean paranoia, thereby significantly increasing chances of accidental war due to miscalculation or misperception.

South Korea would also likely suffer a reputational loss, as well as sanctions, by breaking away from the NPT regime. If South Korea were to test a nuclear weapon, which it would need to do to confirm its capability, the United States would be required under the Glenn Amendment to cut off all military and economic assistance. As one participant noted, South Koreans are very sensitive to the prospect of Americans no longer buying Samsung phones or Hyundai and Kia cars. Furthermore, the moment Seoul announces an intention to pursue nuclear weapons, the Nuclear Suppliers Group would be obliged to halt all provision of fissile material and request a return of anything it had provided to support South Korea’s civil nuclear program.

South Korea’s proliferation is also against U.S. interests. The escalation risks outlined above would diminish stability on the Korean Peninsula and in the region, neither of which is good for the United States. A South Korean nuclear breakout would also further undermine the NPT regime, making it more likely that Japan, Taiwan, and perhaps others would follow in Seoul’s footsteps. This, in turn, would further compound what a participant described as the “window of vulnerability problem” for all these countries, thereby significantly raising the risk of the United States being dragged into a new conflict in the region against its will.

The fourth option is strengthening U.S. extended deterrence. The upsides of extended deterrence are significant: It involves a much lower risk of war than an invasion, does not rely on North Korea’s willingness to engage in good faith to be effective, and avoids the escalation and proliferation risks associated with South Korea’s development of nuclear weapons.

But this option also has downsides. Specifically, changes in the regional security environment and in the U.S. political system have made it more difficult to maintain credible extended deterrence. North Korea’s growing nuclear arsenal requires new demonstrations of capability and will on the part of Washington to impress upon Pyongyang that a nuclear attack would not succeed. The increasing alignment between North Korea, China, and Russia further bolsters North Korea’s capabilities, compounding this challenge for the alliance. Wavering levels of attention to extended deterrence across various U.S. administrations have made the credibility problem worse. Perhaps the most difficult aspect of this lies in convincing the South Korean public that the United States would be willing to risk its own cities to protect Seoul.

For the time being, extended deterrence still presents the best possible approach for the United States and South Korea to pursue their common interests on the peninsula and in the region. Of all the available options, extended deterrence has the most valuable benefits, and its downsides are the most manageable. Devising feasible policy recommendations to bolster the credibility of U.S. commitments requires first obtaining a clearer picture of the current status of extended deterrence credibility on the Korean Peninsula and the closely related issue of South Korean support for the pursuit of nuclear weapons.

Taking Stock of Current Conditions on the Peninsula

U.S. deterrence strategy on the peninsula centers on the ultimate goal of damage limitation, pursued through convincing Kim (or any other North Korean leader) that there is no pathway through which he can achieve his political-military objectives through the use of force, including the use of nuclear weapons. This strategy is upheld by the U.S. pledge embedded in the 2023 Washington Declaration that “any nuclear attack by the DPRK against the ROK will be met with a swift, overwhelming, and decisive response.” If this strategy is successful, the United States will never be put in the position of “trading San Francisco for Seoul,” as the whole aim of the strategy is to never face this scenario.

Making the U.S. threat of “swift, overwhelming, and decisive force” credible has physical and psychological dimensions. Physically, the United States must consistently and visibly demonstrate that it has the capabilities to deter and, if needed, defeat an external attack on South Korea. This aspect is critical, but insufficient, to upholding credibility. Psychologically, North and South Korea must also believe that the United States has the will to devote the full range of its capabilities, including nuclear, to protect Seoul from a nuclear attack. The assurance piece of the equation presents some of the greatest challenges. It requires convincing Seoul, which sits in one of the most acute security environments in the world, that Washington, under increasingly transactional and unpredictable leadership, will live up to these commitments. To paraphrase British Defense Minister Denis Healey’s oft-cited line, nuclear weapons control is 5 percent deterrence and 95 percent assurance.

Physical Dimension: U.S. Capabilities

Overall, the capabilities dimension of extended deterrence is in a solid position. This entails U.S. conventional and nuclear forces (including rotations of U.S. strategic assets), layered missile defense integration, and combined command-and-control (C2) structures, including U.S. forces in Korea and in the region.

U.S. forces play a critical role in forming the core of extended deterrence. The 28,500 U.S. troops stationed on the peninsula ensure that North Korea cannot launch a nuclear or conventional attack on South Korea without killing Americans, thereby guaranteeing a U.S. response. Extended deterrence is most meaningful when the United States has skin in the game, since the troops directly link the conventional and nuclear aspects of U.S. commitments. Regarding the possibility of troop reductions, the actual number is less relevant than the fact that the United States will maintain a forward-deployed presence in South Korea.

Workshop attendees also emphasized the significance of U.S. regional maritime capabilities, including ballistic missile submarines—known as strategic ballistic missile submarines (SSBNs)—low-yield SLBMs, and the commitment to pursue an SLCM-N, which is mandated by Congress to be delivered by 2034 (with the possibility of an earlier delivery). These capabilities bolster deterrence insofar as they are not visible to North Korea and, thus, are survivable. They also provide a persistent deterrent effect, even if they are not always deployed, since Kim has no idea when and where they are lurking.

Institutional frameworks are another strong point. These include the EDSCG, a 2+2 meeting between the ROK and U.S. vice ministers of defense and foreign affairs to coordinate security and policy issues; the Military Committee Meeting (MCM), an annual meeting of the U.S. and ROK chairmen of the Joint Chiefs of Staff; the Security Consultative Meeting (SCM), an annual meeting of the ROK and U.S. defense minister and war secretary; and the NCG, a bilateral body that facilitates senior-level discussions on nuclear and strategic planning.

The NCG—which the Biden and Yoon administrations spearheaded and the Trump and Lee administrations have, so far, continued—represents an evolution in the way the United States has traditionally pursued extended deterrence. In practice, this has involved elevating the role South Korea plays in extended deterrence, enabling it to provide equal inputs into management, governance, and architecture. This, in turn, makes it easier for the United States and ROK to “right-size” the hardware and software needed for both deterrence and assurance at any given moment. It has also allowed for detailed planning on the ways U.S. nuclear and South Korean conventional capabilities would operate together in a range of contingencies. So far, the United States and South Korea have opened 11–12 NCG work streams, including security procedures, communications, detailed planning discussions, tabletop exercises (TTXs), scenario-based discussions, and conventional nuclear integration (CNI). Collectively, the NCG and its affiliated efforts have moved extended deterrence from rhetoric to concrete plans, institutional structures, and embedded exercises.

In addition to the NCG, which convened for the fifth time on December 11, 2025, the Trump and Lee administrations have also continued the work of the MCM, which last met on November 4, 2025, and the SCM, which last met on November 14, 2025. Statements issued after these meetings clarified a new division of labor within the alliance, reiterating the U.S. commitment to provide extended deterrence to the ROK utilizing the “full range of U.S. defense capabilities, including nuclear” while emphasizing that South Korea would assume the leading role in the conventional defense of the Korean Peninsula. While some might see this shift as an indication that the United States is deprioritizing the alliance to focus more on China-related contingencies, it also could be seen as a necessary element of alliance modernization as South Korea’s capabilities improve and the alliance adapts to focus on a broader range of regional threats. The Trump administration’s support for an elevated South Korean role in conventional defense and as a strategic maritime and technological partner was demonstrated by its endorsement of South Korea’s pursuit of nuclear-powered attack submarines, deeper cooperation in the shipbuilding and defense industries, and broader defense industrial linkages in the Indo-Pacific region.

Other notable outcomes from the most recent MCM and SCM include (1) the institutionalization of the linkages between the NCG, CNI, and combined plans, and (2) exercises and the framing of OPCON transition as part of Korean-led conventional defense and alliance modernization (rather than as a decoupling). Overall, these developments have the capacity to strengthen the long-term foundation for extended deterrence through both deepening the alliance and linking it to economic and industrial deals.

Bipartisan support for the alliance in Congress and regular visible demonstrations of deterrence also uphold credibility. Across various U.S. administrations, mechanisms including the National Defense Authorization Act have maintained U.S. troop levels on the peninsula and affirmed the importance Congress places on the U.S.-ROK alliance. Visible displays of U.S. strategic assets, including port calls by SSBNs, joint exercises involving B-52 bombers, and carrier strike group deployments, reinforce both the physical and psychological aspects of extended deterrence.

Despite indications that physical aspects of extended deterrence are sound, there are still concerns in a few areas. The NCG remains in the guidelines phase, not yet taking concrete actions in the field, although much of the work of the NCG takes place out of the public eye, at the classified level. As the work of the NCG progresses, aspects such as bilateral exercises will make progress beyond guidelines more visible.

A key phrase is missing from the November 2025 SCM joint communique. Specifically, the communique does not say, “Any nuclear attack by the DPRK against the United States or its Allies and partners is unacceptable and would result in the end of the Kim regime.” While the intent behind this omission is unclear, it adds to a sense of unease in Seoul regarding the strength of U.S. commitments under the current administration.

Gaps exist between U.S. developments in Joint All-Domain Command and Control (JADC2) and the ROK’s Command, Control, Communications, Computers, Intelligence, Surveillance, and Reconnaissance (C4ISR) backbone. This could affect plans for South Korea to assume wartime operational control of its forces once specific conditions are met, known as a conditions-based operational control transition plan, insofar as the operational structure could be, as described by a participant, “ROK-led in form but U.S.-controlled in substance.” Another concern was the possibility for the OPCON transition timetable to be driven by political developments in South Korea, rather than conditions on the ground (as current agreements mandate).

Seoul also needs to clarify its reasoning behind seeking nuclear-powered attack submarines. Is such a capability necessary to track North Korean or Chinese submarines, given that the distances are not far? Could not the country’s existing diesel submarines manage these types of missions? Or might this be a convenient pathway for South Korea to achieve nuclear sharing (to achieve a survivable capability with U.S. support) or nuclear latency (to achieve, eventually, South Korea’s production of nuclear weapons)? Undetermined details of the deal for the United States to assist South Korea’s development of the submarines, specifically concerning fuel sourcing and the location of production, could create a crisis in the alliance. The degree to which the submarine deal will necessitate a revision of the current U.S.-ROK 123 agreement on civil nuclear cooperation is another point in need of clarification.

Lastly, while the stealth nature of U.S. submarine deployments in the region is an important element of survivability, more visible, tangible measures help to convey a stronger deterrent message to North Korea and assure South Korea of U.S. capabilities and will. The redeployment of U.S. tactical nuclear weapons to the peninsula could be one measure that increases the visibility and tangibility of U.S. commitments, though this would also entail considerable downsides (discussed below). The December 24, 2025, announcement that the United States and South Korea agreed to produce a stand-alone pact to guide cooperation on nuclear-powered submarines indicates that Washington and Seoul have started to address some of these questions.

What Deters Kim Jong-un?

Discussions concerning extended deterrence often do not directly tackle an essential question: What specifically deters Kim Jong-un? Addressing this question is critical to devising deterrence strategies and messaging insofar as leaders seek to publicly tout the effectiveness of their policy decisions. But the opaque nature of the North Korean regime makes finding a definitive answer to this question inherently difficult. One may assume, for instance, that the U.S.-led “maximum pressure” campaign of 2017–18 deterred North Korea from launching a nuclear attack and brought Kim back to the negotiating table. This is certainly possible, but it cannot be confirmed absent evidence that Kim intended to launch an attack at that time. The same applies to rotations of strategic assets. If the United States sends a carrier strike group or nuclear-capable aircraft to the peninsula and no North Korean attack ensues, one may assume North Korea is being deterred. But the possibility also cannot be ruled out that North Korea would not have launched an attack in any case.

In the absence of a smoking gun that would confirm that deterrence explains the absence of attacks while ruling out other possibilities, the United States and South Korea must proceed under the assumption that North Korea is ready and willing to attack at any moment. If they do not, military readiness in the alliance could wane, potentially causing North Korea to see a window of opportunity to strike. As a result, attempts to discern what deters Kim—coupled with efforts to gather as much information as possible regarding North Korean intentions and capabilities—remain important, even if clear answers remain elusive.

Psychological Dimension: Assuring South Koreans of U.S. Willingness to Use Its Capabilities

The “assurance problem” has been a perennial challenge in the U.S.-ROK alliance that is unlikely to ever be fully resolved. Levels of assurance are extremely difficult to measure in concrete terms, as they by nature entail subjective perceptions.

One workshop participant suggested that devising a “credibility dashboard” to assess the strength of extended deterrence over several U.S. administrations may help to overcome these difficulties. According to this framework, three dimensions are fundamental to upholding the physical and psychological aspects of extended deterrence: (1) clear and consistent U.S. expressions of political will to defend South Korea, in both words and actions, (2) concrete military options and execution capabilities, and (3) communications and public exposure, which are important because, as one participant put it, “even if political will and capabilities exist, if allies and citizens do not see and feel them, trust will not grow.” Key indicators in these areas include the frequency and strength of top-level statements, the pattern and visibility of strategic deployments and combined exercises, and polling trends in South Korea. The results of this assessment, displayed in the box below, indicate the degree to which assurance problems remain a central challenge but are far from a new feature in the alliance.

Credibility Dashboard: Assurance Challenges over Time

The credibility dashboard, applied to the last three U.S. administrations, as well as the current one, yielded the following observations:

• The Obama administration maintained political will and deterrence capabilities, but the broader nuclear disarmament agenda caused many in Seoul to feel a certain psychological unease. Questions lingered about how lowering the role of nuclear weapons, in the long run, does not also lower the political will behind the nuclear umbrella.

• During the first Trump administration, core military capabilities remained intact, but political will and communications were on what felt like a roller coaster. The administration moved from fire and fury and real fears of war to summits in Singapore and Hanoi, scaling down major combined exercises and placing strong pressure on Seoul for special measures agreement contributions.

• Under the Biden administration, focus shifted to alliance restoration and institutionalization, with capabilities bolstered by new initiatives including the NCG and communications becoming more consistent. But the absence of U.S. troop commitments to the Ukraine war, the abandonment of Afghanistan, and ambivalence toward foreign engagements in general continued to present a structural risk.

• In the current Trump administration, although capabilities and structures are being strengthened, political will has been communicated in a transactional and fluctuating way. Summits, joint statements, strategic deployments, and combined exercises have sent strong and visible signals of commitment, but public statements about tariffs, burden sharing, and fee-procured deals are reported just as widely in the Korean media. As one participant noted, “Koreans are, in fact, hearing two messages at the same time: We will strengthen the alliance and enhance your security, but this is contingent on the right deal, the right price, and the right reciprocity.”

Some might look at this record and measure a moderate level of psychological trust in U.S. commitments among South Koreans. This level may be acceptable, as no ally in the history of U.S. extended deterrence has ever rated credibility high. Both allies need to acknowledge the inherent attribution dilemmas at play when it comes to psychological levels of trust. Allies often see actions that the United States takes in support of deterrence and reassurance as situational and dictated by expediency. Meanwhile, South Korea interprets actions not taken by the United States in support of deterrence as being dispositional, or a signal of true intentions. Acknowledging these cognitive-psychological dynamics can foster better understanding and reduce anxieties and frustration in the alliance.

South Koreans also see some benefits from transactionalism, noting that President Trump’s vision and understanding of South Korea, as well as Japan, have shifted significantly since his first term. During his first administration, President Trump viewed these countries as drains on U.S. resources that did not contribute to strategic interests. Over the past five years, however, engagement and investment in the United States by South Korea’s government and companies have led Trump to see South Korea as an advanced economy that is essential to reshoring and building out U.S. industrial might. As evidence of this, President Trump said yes to Lee’s request for nuclear submarines—something the United States would not have historically agreed to—presumably because he sees this as an element of his broader strategy. Through this lens, the South Korean public can see that a dealmaking approach to alliance management, which has brought new forms of economic and security pressure, also involves opportunities and advantages for Seoul that were not previously anticipated. In this way, the degree to which transactionalism will reduce levels of assurance remains undetermined.

Support for Nuclear Weapons in South Korea: An (Imperfect) Inverse Proxy for Assurance

Public polls can also serve as an indicator of levels of South Korean trust in U.S. extended deterrence commitments. Specifically, polls that gauge public support for South Korea’s pursuit of indigenous nuclear weapons can provide insight into the status of U.S. assurance. High levels of domestic support for South Korea carrying its own nuclear weapons reflect a perception that the U.S. assurance of extended deterrence is insufficient. It is important to note, however, that this is an imperfect proxy, as factors such as public support for greater South Korean autonomy in providing for its own defense, North Korean provocations, and the potential for nuclear proliferation in the region can also affect support levels for South Korean nuclear ambitions.

Two recent sets of polls reveal a significant gap between the way the public and strategic elites in South Korea view the idea of their country going nuclear. A March 2025 Asan Institute survey of 1,000 South Koreans indicated that a record 76.2 percent of respondents among the general public supported indigenous nuclear armament (an increase from 70.9 percent in 2024). Meanwhile, confidence that “the United States would use nuclear weapons to defend South Korea” remained largely unchanged since the prior year at 48.9 percent. Support for the redeployment of U.S. tactical nuclear weapons also went up to 66.3 percent, the second-highest level since 2013.

A 2024 CSIS survey sent to over 1,000 South Korean strategic elites revealed that only 34 percent of respondents held positive views about nuclear weapons development, while 66 percent did not have a positive view (i.e., were either negative or uncertain about South Korea going nuclear). Notably, the 66 percent of respondents who were not positive about indigenous nuclear weapons represented a wide swathe of the South Korean political spectrum, including progressives, moderates, and conservatives. This group also indicated sensitivity to changes in U.S. political leadership: 83 percent of elites who were uncertain about South Korea going nuclear and 51 percent of elites who were not supportive of nuclearization said they would become more supportive if a leader espousing an “America First” agenda returned to the U.S. presidency.

When CSIS conducted a second poll of strategic elites in July–October 2025 for a forthcoming report, this time in both South Korea and Japan, the percentage of South Korean strategic elites who were not in favor of or ambivalent about going nuclear went up from 66 percent to 74 percent. Meanwhile, the number in favor of going nuclear went down almost 10 points to 25 percent.

▲ Figure 1: Do you agree with the following statement? “South Korea should have nuclear weapons.”

▲ Figure 2: If an “America First” policy returns to the White House in November 2024 that denigrates allies and seeks retrenchment, your support for a nuclear South Korea would: ~

▲ Figure 3: Do you agree with the following statement? “South Korea should have nuclear weapons.”

These results are surprising, given the responses in 2024 indicating that support for South Korea’s nuclearization would increase if an America First president returned to office.

There are several potential drivers of this decrease in strategic elite support for South Korea’s nuclearization, some more complementary than others:

• First, the Washington Declaration and NCG may be effective in increasing the credibility of extended deterrence. This is consistent with the observation of one participant that “after the Washington Declaration was signed, it was like a light had been switched. . . The Korean government stopped talking about going nuclear.”

• Second, the reality of America First foreign policy may be sobering. Now that it is real, the costs of South Korea’s pursuit of nuclear weapons also may seem more severe.

• Third, and related to the second explanation, strategic elites may have gained a better appreciation of the costs of South Korea going nuclear over the past year, particularly since Trump entered office.

• Fourth, South Korean elites may have confidence in President Trump, perhaps focusing on the actions he has taken to bolster alliance capabilities (though this survey was conducted in July–October 2025, before the security aspects of the relationship had been thoroughly discussed by the new administrations).

• Finally, related to the third explanation, strategic elites may not have confidence that Trump would acquiesce to South Korea going nuclear. Despite Trump’s past statements, such as suggesting during the 2016 campaign that South Korea, Japan, and other countries should go nuclear, South Korean elites may assess that the president’s views have shifted as he has started to view Seoul and Tokyo more as strategic partners in the region.

While the drivers of decreased strategic elite support for South Korea’s nuclear ambitions in the 2025 poll remain ambiguous, the poll presented a clearer image of the impact of the so-called cascade or domino effect in the region, with strategic elites in both South Korea and Japan expressing that they would increase support for their own country acquiring nuclear weapons if the other country took that path.

Specifically, among strategic elites in Japan, the percentage of respondents who were either ambivalent about or against Japan going nuclear was quite high, around 79 percent. Yet, given Japan’s nonnuclear culture and principles, it is still significant that around 21 percent responded positively. Perhaps more significantly, when the group against nuclearization (64 percent) was asked whether their views on Japan going nuclear would change if South Korea acquired nuclear weapons, 63 percent said they would be in favor. For those in the ambivalent group (15 percent), 77 percent said they would shift to support Japan’s pursuit of nuclear weapons if South Korea went nuclear.

When the same prompt was presented to the group of South Korean strategic elites who were not initially in favor of South Korea getting nuclear weapons (“If Japan acquires nuclear weapons, your support for a nuclear South Korea would increase”), 79 percent agreed. For those who were not sure if South Korea should get nuclear weapons, 90 percent said they would support nuclearization if Japan nuclearized.

Collectively, these results affirm that levels of trust among South Koreans in the U.S. commitment to extended deterrence—while certainly a significant factor affecting support for South Korean nuclear weapons—are not the only driver of this support. Among other things, nuclear breakouts in neighboring countries could also increase public support for South Korea’s pursuit of nuclear weapons.

Policy Recommendations for Optimizing Extended Deterrence

Although the credibility of U.S. extended deterrence commitments will likely never be absolute, certain moves can predictably increase or decrease belief in U.S. capabilities and will to protect South Korea. Credibility is not created by a single declaration; it is a habit built over time and a collective memory formed by repeated and shared actions. Habits and collective memories formed over the next few years will determine whether the trajectory of credibility curves upward into a scenario of greater stability and higher trust, remains sturdy but oscillating (as it is today), or descends into a downward spiral of lower trust, which would likely prompt South Korea to seek its own nuclear weapons capabilities. The recommendations below outline practical steps U.S. and ROK policymakers can take to strengthen extended deterrence credibility and keep its trajectory moving upward.

Building on Existing Institutional Capacity

Further strengthen and institutionalize U.S.-ROK extended deterrence mechanisms, including the NCG and EDSCG.

• Ensure that the NCG and EDSCG remain adequately staffed and resourced.

• In the face of criticism about the NCG’s impact, remind skeptics that the NCG was designed to continually evolve to meet current and future threats affecting the U.S.-ROK alliance.

• Maintain adaptiveness, including the ability to set up new TTX scenarios based on where strategic and tactical developments are heading.

• Continue to build cooperation on CNI and joint planning efforts, including through close consultation between the newly established K-STRATCOM, United States Forces Korea, and the Combined Forces Command.

• Raise the priority of difficult conversations within the NCG regarding nuclear use scenarios, including on the role of civil defense, civilian protection, and combatant evacuation operations in a nuclearized environment.

• Share information with the U.S. and South Korean publics about developments within the NCG and EDSCG whenever possible to increase levels of awareness and trust in U.S. extended deterrence commitments.

Multilateralize strategic dialogues, initiatives, and exercises.

• Consider developing an overarching regional dialogue on extended deterrence that connects and coordinates all the regional U.S. extended deterrence mechanisms, including the NCG with South Korea, the Extended Deterrence Dialogue (EDD) with Japan, and the Strategic Policy Dialogue (SPD) with Australia.

  • This proposed dialogue is particularly important given the fact that any nuclear employment by China or North Korea in the Indo-Pacific would implicate all U.S. extended deterrence partners. In such a scenario, having a common operating picture of what occurs at the NCG, EDD, and SPD, and coordinating responses, would be very beneficial.

  • This type of coordination would also generate efficiencies that are likely to appeal to the Trump administration.

• Initiate a trilateral U.S.-Japan-ROK dialogue on extended deterrence.

  • The annual STRATCOM meeting in Omaha, Nebraska, could provide an ideal venue for this type of discussion.

  • This move would take advantage of the degree to which the Lee and Sanae Takaichi governments seem to share a core understanding of their need for each other.

  • These developments would align well with President Trump’s conception of South Korea and Japan as strategic partners.

  • This type of discussion would be timely, given Japan’s attention to South Korea’s nuclear debate and its own enlivened conversations about nuclear weapons.

• Consider adding Australia to some U.S.-Japan-ROK security discussions and initiatives.

  • Adding “cooks to the kitchen” can involve downsides but also certain upsides if political dynamics are difficult among the original three members.

• Consider initiating regional nuclear arms and missile limitation talks, given the possibility that China and Russia might be interested in deploying intermediate-range ballistic missiles in the region.

  • Even though South Korea and Japan are not nuclear powers, they could be interested in joining these talks insofar as South Korea has missile capabilities and Japan could be the victim of missile attacks.

• Explore security commitments below the nuclear level with partners outside the neighborhood.

  • Cross-regional conversations with NATO about extended deterrence and other forms of security and defense industrial cooperation would be useful.

  • Briefings on DPRK-Russia cooperation provide a natural starting point for broader conversations between South Korea and NATO on shared interests in Indo-Pacific stability.

  • Reviving the Indo-Pacific Four (IP-4) would further bolster these initiatives.

• Energize multilateral Track 1.5 dialogues on extended deterrence.

  • Among other things, this would give the South Korean and Japanese strategic communities a better sense of developments in the United States.

  • This could also be a useful forum for intraregional extended deterrence dialogues, as well as cross-regional conversations between Asian and European allies.

• A potential downside is that multilateralization in any form might push China, Russia, and North Korea closer.

Maintaining U.S. Forces on the Peninsula

When considering alliance modernization efforts, including wartime OPCON transition, allies should keep in mind the critical role that U.S. troops on the peninsula play in upholding credible extended deterrence.

• The actual number of forces is less important than the degree to which U.S.-ROK combined capabilities, cooperation, and alignment remain steady.

Closely Coordinating and Managing Expectations Regarding the U.S.-ROK SSN Deal

Establish U.S. interagency, U.S.-ROK bilateral, and Track 2 processes to clarify details of the SSN deal and avoid a crisis in the alliance.

• Issues related to fuel supply, the location of production, and the relationship between the SSN deal and the U.S.-ROK 123 agreement involve thorny issues that could create long delays and fresh tensions in the alliance if not managed carefully.

• Future U.S. administrations may not support the SSN deal or be able to fund the project, making it important to resolve as many details as possible while President Trump is still in office.

• Track 2 discussions can increase awareness on both sides of the practical constraints involved in SSN design and construction.

Considering Pre-Decisional Discussions on U.S. Tactical Nuclear Weapons Redeployment and Nuclear Sharing

The redeployment of U.S. tactical nuclear weapons to the Korean Peninsula could involve several advantages.

• Redeployment would provide a credible, visible linkage between U.S. retaliatory capabilities and ROK conventional security, thereby assuring South Korea of U.S. willingness to provide protection.

• It would provide an additional visible capability to intimidate North Korea. By making U.S. nuclear forces physically present and unmistakeable, such visibility would complicate Pyongyang’s escalation calculus, raising the perceived costs and risks of any nuclear or conventional provocation at an early stage of decisionmaking.

• Keeping this option open presents a potential means to apply pressure on North Korea in negotiations. In the absence of any U.S. nuclear weapons deployed in South Korea today, Pyongyang has little incentive to contemplate meaningful nuclear reductions; forward deployment could therefore serve as a concrete bargaining lever, creating pressure for reciprocal restraint or arms control measures rather than unilateral concessions by the alliance.

• Making U.S. nuclear commitments more physically present on the peninsula could dampen domestic calls for an independent South Korean nuclear arsenal.

There are also considerable downsides to U.S. tactical nuclear weapons redeployment.

• During the Cold War, when North Korea did not have nuclear weapons, forward-deployed U.S. nuclear weapons provided assurance and deterrence without being vulnerable. In contrast, forward-deployed U.S. nuclear weapons today would be extremely vulnerable because of the geographic proximity of North and South Korea. By comparison, NATO countries have large buffers between Russia and forward-deployed nuclear weapons that reduce their vulnerability.

• Because of the vulnerability of forward-deployed tactical nuclear weapons on the peninsula, these could be preempted before the United States and South Korea can defend against a potential nuclear attack.

• These dynamics could incentivize North Korea to go nuclear even faster.

• The storage of nuclear weapons on the Korean Peninsula would also involve serious costs and constraints in terms of infrastructure, training, and personnel. Furthermore, most South Koreans do not want to store nuclear weapons in their backyard.

The introduction of a nuclear sharing mechanism would also involve significant complications.

• Certifying dual-use aircraft and submarines is quite difficult and cannot be achieved on a short-term emergency basis.

• Certified ROK F-35As and U.S. forward-deployed tactical nuclear weapons would not avoid the vulnerability problem insofar as they are visible and geographically close to North Korea.

• Furthermore, any ROK F-35As certified for nuclear delivery would have to be held in reserve in any conventional conflict to avoid being attrited and therefore becoming unusable in a potential nuclear strike mission.

The SLCM-N can provide a powerful deterrent that is less vulnerable to preemptive attacks than forward-deployed U.S. tactical nuclear weapons on the peninsula.

• SLCM-Ns are more survivable because they are a standoff capability that is not visible to the adversary.

• Low visibility also entails a downside, as it is less tangible to the South Korean public and therefore has a weaker assurance effect.

Periodic deployments of strategic assets to the peninsula continue to serve as an important reminder of U.S. capabilities.

• The key is making these deployments regular, not a response to specific events only.

Seeking to Establish Crisis Management Mechanisms and Reduce Tensions with the DPRK Without Weakening the Alliance or Deterrence Capabilities

The United States and South Korea should establish a forum to discuss and coordinate North Korea’s policy positions with each other before any meetings with Pyongyang.

• This will ensure that North Korea fails in any divide-and-conquer strategies to turn the allies against one another.

• Pre-DPRK engagement discussions between the allies should include coordination on any potential peace declaration with North Korea.

Negotiations with the DPRK should maintain denuclearization as a stated aim but be realistic about its low prospects.

• Abandoning denuclearization as a stated long-term aim of the United States and South Korea would risk spurring regional proliferation.

• For the same reason, the United States and South Korea should avoid using terminology that creates the impression that they recognize North Korea as a nuclear power.

• The talks could entail a more limited aim of a testing moratorium, which would, at least, slow the further development of North Korean capabilities.

Efforts to reduce tensions with the DPRK should avoid steps that would affect deterrence, such as suspending U.S.-ROK military exercises.

• If negotiations fall apart, the alliance needs to remain in a strong position to deter future North Korean aggression.

• The terms of any potential peace declaration should avoid undermining the strength of the alliance or military readiness.

• Maintaining deterrence and dialogue simultaneously (which can seem like an impossible task involving creating pressure without tension) will require thinking creatively to develop new solutions to this age-old dilemma.

Devising a Public Communications Strategy to Increase Knowledge About Nuclear Issues

Form a joint nuclear options study group to increase public awareness of the pros and cons of various nuclear strategies.

• There is strong public support in South Korea for strengthening capabilities to deter North Korea’s nuclear threat, but public education and discussions of the pros and cons of various options—including South Korean indigenous nuclear weapons development, redeploying U.S. tactical nuclear weapons, and nuclear sharing—are inadequate.

• Public opinion polls indicate that South Korean support for nuclear weapons or tactical nuclear redeployment decreases when information about the potential downsides of these moves is shared. Ensuring the public receives comprehensive information on these options is therefore critical to ensuring that politicians do not alter their positions based on pressures from an uninformed public.

Devise initiatives to increase nuclear literacy among the public without instilling panic and fear that create new assurance dilemmas.

• Inform the public about the dangers of nuclear war without scaring them so much that they demand increasing (and excessive) levels of protection.

• A good place to start might be an education campaign about the basics of extended deterrence, as well as what the NCG is and does, since much of its work is in the classified domain. This could help to increase levels of assurance, which, in turn, would likely dampen support for South Korea’s nuclear ambitions.

• Movies such as House of Dynamite, which are not entirely accurate, provide an opportunity to engage and educate the public on what more realistic scenarios could look like, as well as the degree to which allied cooperation could help to alleviate threats in these scenarios.

• Providing public readouts of Track 1.5 and Track 2 discussions, and possibly TTXs, is another option.

Consider sharing U.S. space-based early warning data with South Korea and Japan following North Korean missile tests to ensure assessments of the three allies are on the same page.

• Circulating divergent assessments on the number and trajectory of missiles in regional media creates impressions among the public that deterrence mechanisms are inadequate.

Prioritizing Careful Alliance Management, Especially as the United States and South Korea Face Evolving Domestic Pressures and Regional Challenges

Seoul should be cautiously proactive in communicating its sensitivities and concerns with Washington related to developments in the region and within the alliance.

• In doing so, Seoul should continue to frame its messages in ways that are compatible with this administration.

• Considering the rapport that President Lee has developed with President Trump, his administration is off to a good start in this regard.

The United States, for its part, should value political consultations with South Korea as a way to optimize its investments in the alliance.

• As one aspect of this, Washington should be empathetic to sensitivities that were inflamed in South Korea following the Georgia raid and work to avoid similar incidents in the future.

Together, Washington and Seoul should consider a new framework of expanded deterrence, taking into consideration not only Pyongyang’s capabilities but also those of Beijing and Moscow.

• Such a framework should consider the potential for North Korea or regional powers to use nuclear weapons in nontraditional ways, such as by instigating an electromagnetic pulse (EMP) attack.

• The two allies should also be open to considering a one-theater concept—merging different combat theaters across the Indo-Pacific into a single, coordinated area of wartime coverage— that accounts for potential linkages among contingencies in the region.

Enduring Extended Deterrence Dilemmas: Striking the Right Balance

Embedded in the challenges of maintaining credible U.S. commitments across physical and psychological dimensions are several unique extended deterrence dilemmas that require policymakers to seek an optimal balance between competing objectives.

Dilemma 1: Navigating Tensions Between Credibility and Acceptability

• Enhancing deterrence via new capabilities can sometimes introduce new risks that the allied public does not find acceptable. For instance, the redeployment of U.S. tactical nuclear weapons to the Korean Peninsula would add a visible linkage between U.S. retaliatory capacities and South Korean conventional security. But it also increases the vulnerability of South Korea to a strike on the storage sites, which nobody wants in their backyard (the “NIMBY” phenomenon).

• A key question for policymakers is: What capabilities can enhance deterrence in ways that are acceptable to the South Korean public?

Dilemma 2: Ensuring that Deterrence Does Not Create Assurance Problems, and Vice Versa

• Some steps that enhance physical deterrence versus an adversary may not be visible or captivating enough to assure the ally. For instance, the introduction of U.S. SLCM-Ns in the region would enhance deterrence insofar as they are a new capability that could be deployed offshore, are not visible to North Korea, and are therefore survivable. But the South Korean public also would not be able to see the SLCM-Ns and would have no idea when and where they would be deployed. Would knowing that they are out there somewhere be enough to provide assurance?

• On the other side, steps that may assure the ally may not necessarily register as enhancing deterrence against the adversary. For instance, the frequent and consistent repetition of the U.S. pledge to dedicate “the full range of its military capabilities, including nuclear” to protect South Korea from external attack is deemed essential to assuring South Koreans.33 But talk can be cheap. Do these statements actually deter North Korea?

• Steps to reassure the ally can also create deterrence problems for the opposite reason by increasing the risk of escalation. The reintroduction of U.S. tactical nuclear weapons could provoke this dilemma. While this provides a more visible, concrete manifestation of U.S. capabilities that can help assure South Koreans, it also presents new targets for North Korea, and possibly China and Russia, as well as new incentives to strike.

• A key question for policymakers regards how to navigate trade-offs between deterrence and assurance: Is there a sweet spot of capabilities that can both assure the ally and deter the adversary?

Conclusion

The Importance of Strong Alliance Cooperation and Leadership amid Structural Change

The pressures on U.S. extended deterrence involve a layering of dynamics at two levels. At one level, long-standing structural developments related to the growth of North Korea’s capabilities started as far back as North Korea’s 2003 withdrawal from the NPT and first nuclear test in 2006. The intensification of U.S.-China geopolitical competition is also an area of structural change in the region that has directly affected dynamics on the Korean Peninsula across various administrations.

On another level, leadership decisions clearly matter in deftly navigating these structural shifts, perhaps now more than ever. Accordingly, decisions made by the Trump and Lee administrations in the next three to four years will play a key role in restoring the credibility of extended deterrence with implications for the future viability of the U.S.-ROK alliance, regional proliferation, Indo-Pacific stability, and the likelihood of nuclear war.

Victor Cha is president of the Geopolitics and Foreign Policy Department and Korea chair at the Center for Strategic and International Studies (CSIS). He is also a distinguished university professor and professor of government at Georgetown University.

Choi Kang is the president at the Asan Institute for Policy Studies. Previously, he was the dean of planning and assessment at the Korean National Diplomatic Academy.

Katrin Fraser Katz is an adjunct fellow (non-resident) in the Office of the Korea Chair at CSIS.