Seoul’s Nuclear Temptations and the U.S.-South Korean Alliance
Amid drastic negative changes to its security environment and fundamental questions about the long-term reliability of the United States, South Korea is drawn — as it once was in the 1970s — to nuclear weapons. On Jan. 11, South Korean President Yoon Suk-yeol, a conservative who has been outspoken about North Korea’s nuclear threats, voiced the possibility that Seoul could “acquire our own nuke.” Alluding to his country’s advanced scientific prowess, delivery systems, and long-acknowledged nuclear latency, Yoon noted that, should such a decision be made, Seoul’s advanced “science and technology” would ensure that the time required to build such a capability would be short. Yoon’s words have made global headlines and jolted alliance hands in Washington.
Yoon’s remarks, while concerning, do not represent the result of a considered policy planning process or indicate that a decision to procure nuclear weapons will be soon made in South Korea. Instead, the South Korean president alluded to the possibility of pursuing nuclear weapons in a wide-ranging set of remarks to South Korean foreign and defense officials. According to one unofficial translation of a released Korean transcript, Yoon premised the possibility of nuclear weapons acquisition on a conditional: “if problems become more serious,
[South Korea] could deploy tactical nuclear weapons here, or we could acquire our own nuke as well.” He concluded this section of his remarks by reverting to the status quo, noting that the “realistically possible” option, “for now,” was the alliance with the United States. In the days since his remarks went public, Yoon has tried to manage perceptions: for instance, Yoon publicly noted that South Korea’s “realistic and rational option is to fully respect the [Non-Proliferation Treaty] regime” and that he remained “fully confident about the U.S.’s extended deterrence.”
South Korea faces difficult choices amid an objectively worsening threat environment, but a drastic shift away from the status quo of robust conventional deterrence backstopped by U.S. nuclear guarantees in pursuit of an independent nuclear deterrent will not solve South Korea’s security challenges. Beyond the normative, economic, and other costs Seoul would face for abrogating its non-proliferation commitments, it is far from clear that South Korean nuclear weapons will help solve tensions with North Korea. As anxieties remain high, Washington and Seoul should refocus their efforts on adapting their military plans to a shifting North Korean threat while exploring new forms of trust-building within the existing alliance structure. This will require coordinating views on what North Korean behaviors can be deterred and through what means and working together to assuage South Korean concern about U.S. security guarantees.
Inter-Korean Crises and Proliferation Concerns
Given how rare publicly expressed statements of nuclear weapons acquisition intent are by U.S. extended deterrence recipients in the post-Cold War era, Yoon’s comments are unlikely to be ignored in Washington. The comments represent an expression of a view that is deeply held by many in South Korea, including among officials within the current government. Many prominent advisors in the Yoon administration served in the Lee Myung-bak administration, including during the turbulent years of 2008 to 2010, when Pyongyang became exceptionally risk acceptant. Inter-Korean tensions crescendoed with the twin crises of 2010, when North Korea sunk the ROKS Cheonan, a South Korean Navy corvette, and later shelled Yeonpyeong Island across the Northern Limit Line. Between civilians and military, 50 South Koreans lost their lives. South Korea was prepared for war, but the Obama administration discouraged disproportionate escalation by Seoul in the pursuit of vengeance, fearing uncontrollable consequences. These experiences have informed the approach of some in the Yoon administration today who are determined to never allow for a 2010-style crisis to repeat itself and see nuclear weapons as an instrumental component of deterring a range of undesirable actions by North Korea.
2010 was a dramatic reminder that the kind of risk-acceptant behavior North Korea exhibited in the post-Korean War period under Kim Il Sung — as seen in prominent crises throughout the 1960s and 1970s — was not a relic of the past. In the pre-democracy period, South Korea, under Park Chung-hee, considered developing nuclear weapons once and was coerced away from that path and into the then-nascent non-proliferation regime by the United States. Since Kim Jong Un’s assumption of power in the final days of 2011, the Korean Peninsula has been spared serious skirmishes, but as the final days of 2022 demonstrate, Pyongyang continues to surprise with behavior that could precipitate unintentional escalation. Kim Jong Un, meanwhile, continues to oversee a massive project of quantitative nuclear force expansion and qualitative modernization. Seoul’s threat perceptions are well-placed and should not be dismissed: In fact, insofar as a state’s security environment can drive an interest in nuclear weapons acquisition, the South Korean case should not be a surprise.
Do Nuclear Weapons Solve South Korea’s Security Dilemma?
It is far from clear that nuclear proliferation will help Seoul solve the security problems it perceives today. As Yoon himself has noted since his published remarks on nuclear weapons acquisition, the United States and South Korea can continue to rely on their alliance and even primarily on non-nuclear capabilities to effectively deter North Korean nuclear use in the service of large-scale territorial revisionism on the Korean Peninsula. What nuclear weapons — American or South Korean — won’t do is solve the general nuisance that Pyongyang has been and remains for South Korea.
“Provocations,” the preferred South Korean term for any number of North Korean actions that Seoul finds disagreeable, will not be coerced away by manifesting nuclear weapons — either American or South Korean — more clearly on the Korean Peninsula. As nuclear-armed India has discovered with its own territorially contiguous, nuclear-armed neighbor Pakistan, nuclear weapons are lousy solutions to subconventional threats and even limited territorial aggression. (Israel’s covert nuclear arsenal, similarly, has hardly solved the problem of limited and subconventional war.) Policymakers in Seoul are understandably searching for solutions that can drastically improve their security in the face of decisively negative trendlines concerning North Korea’s capabilities, but nuclear weapons are unlikely to provide answers. On the Korean Peninsula, South Korea and the United States should remain focused on the overarching task of deterring large-scale war and, most importantly, nuclear use. To do this, the alliance must take stock of the full array of capabilities already available — nuclear and conventional — and adapt to the reality of North Korea’s nuclear threat through updating their plans, procedures, and strategies. Washington and Seoul must also develop a shared vision of what exactly the alliance seeks to deter: For instance, missile tests and other shows of force in peacetime may be objectionable — but ultimately undeterrable.
U.S. Nuclear Options for Korea
In his remarks, right before Yoon mooted proliferation, he raised the prospect of the United States redeploying tactical nuclear weapons in South Korea. Yoon’s push for the return of these weapons is not surprising. It was a campaign pledge when Yoon was a candidate for the presidency. However, cabinet-level officials in his administration, including Defense Minister Lee Jong-sup, subsequently appeared to rule out this option in unequivocal terms. The United States removed its last tactical nuclear weapon from South Korea in December 1991 as part of a broader global nuclear drawdown pursuant to the Presidential Nuclear Initiatives between U.S. President George H.W. Bush and Soviet leader Mikhail Gorbachev. Incidentally, the departure of these weapons took place days before the two Koreas first agreed to jointly pursue “denuclearization of the Korean Peninsula.” Today, the demand for nuclear reassurance in South Korea is greater than ever — borne of a growing belief in Seoul that only nuclear weapons can deter North Korea’s nuclear use.
Unlike in 1991, the array of so-called “tactical” nuclear capabilities available to the United States today for putative forward deployment are limited to fighter and bomber-delivered variable-yield B61 (initially, mod 3 and mod 4, and eventually mod 12) gravity bombs. These weapons could be redeployed should a political decision be made in Washington to meet Seoul’s demands. The military utility of these weapons on the Korean Peninsula, however, is questionable. If forward-based on the Peninsula, as some in Seoul seek, B61s would almost certainly be fighter-delivered by F-15E or F-16C/D aircraft. (Once available, the B61 mod 12 will be deployable on certified F-35A fighters.) In a crisis, North Korea would have strong incentives to prioritize airfields hosting these aircraft and associated B61 infrastructure for preemptive strikes. Pyongyang has indicated that it views airfields that host advanced aerial capabilities, including South Korean Air Force F-35As, as prime targets for preemptive strikes. The promptness with which North Korea could release precise, solid propellant ballistic missiles against these airfields would be substantial: The time for these aircraft to receive B61s and take off would be greater. These problems remain even if the United States were to adopt a nuclear-sharing style arrangement where South Korean pilots and aircraft could be certified to deliver the B61.
There are other complications for B61 deployment. Seoul would need to find suitable bases to host U.S.-made Weapons Storage and Security System vaults, which would ensure that these weapons would be safely and securely stored. The placement of these vaults may encounter fierce local political opposition in South Korea, further stressing the alliance: The reaction to the deployment of the THAAD battery in Seongju is telling. Even if weapons are not deployed today, these vaults could be built and B61s could be rotated in during a crisis. This, however, would be perceived as a highly escalatory action by North Korea, which could see incentives to preempt storage sites and airfields prior to the arrival of these weapons. There are few certified aircraft that can transport B61 gravity bombs and it would be likely that, even if transport were undeclared, open-source techniques could identify potential delivery of these weapons in a crisis, potentially precipitating North Korean escalation. (Potential North Korean human intelligence assets in South Korea could similarly surveil deliveries.)
If B61s are to have any role in this ongoing reassurance debate within the alliance, it would be to serve, as former Obama administration official Elaine Bunn memorably put it, a “wedding ring” of reassurance. In effect, once deployed, the nuclear bombs could serve as a symbolic show of commitment to Seoul’s defense — even if at best their military use would be limited and, at worse, actively destabilizing. This is the least compelling rationale for nuclear weapons deployment and the balance of evidence does not suggest that on-Peninsula B61s will be a panacea to the problems posed by North Korea. The “wedding ring” rationale helps explain why U.S. B61s, having been deployed in Europe, are unlikely to be removed, lest their departure be seen as a sign of American infidelity.
B61s ultimately receive disproportionate attention in ongoing debates in Seoul. While the United States maintains a smaller array of lower-yield capabilities — what some might consider “tactical” — than it did at the height of the Cold War it does have other options to address Seoul’s demands for credible nuclear reassurance. One other air-deliverable weapon is the AGM-86 air-launched cruise missile, which includes a primary-only 5 kiloton yield option. These can be carried and launched by B-52H strategic bombers from stand-off ranges at any target in North Korea. As a subsonic delivery system launched from a subsonic bomber, with warheads stored away from South Korea, the air-launched cruise missile would lack promptness in a crisis, however.
A potentially attractive option on the nuclear reassurance menu — one that U.S. officials appear to underemphasize in ongoing extended deterrence consultations — is the submarine-launched, low-yield Trident D5 missile. This capability was introduced by the Trump administration’s 2018 Nuclear Posture Review for reasons nominally having nothing to do with North Korea and has been deployed on Ohio-class ballistic missile submarines since at least February 2020. These missiles would be capable of delivering a low-yield (assumed between 5-10 kiloton) nuclear warhead to any site in North Korea with exceptional promptness. It is reasonable to assume that at least one U.S. Ohio-class submarine is within striking range of North Korea at all times — 24 hours a day, 7 days a week, and 365 days a year. In some cases, an Ohio-class ballistic missile submarine positioned favorably in the Pacific Ocean would be able to strike North Korea with a D5 missile in approximately 20 minutes. This is comparable to the time-to-target the United States enjoyed with certain tactical nuclear weapons that were deployed to the Korean Peninsula between 1958 and 1991 but no longer exist in the U.S. arsenal.
This option is undesirable for a variety of reasons unrelated to South Korea, specifically. For instance, Trident D5 launches could raise the risk of inadvertent nuclear war with Russia, which may be unable to discriminate a low-yield Trident from one carrying a higher-yield nuclear warhead. Depending on the position of a given Ohio-class submarine, a North Korea-bound missile may need to overfly Russian territory. Russian early warning systems may also interpret a North Korea-bound missile as bound for its own territory, prompting Moscow to consider firing back at targets in the United States or Europe (especially during a time of crisis in Ukraine). Secondly, as the most survivable leg of the U.S. nuclear triad and America’s secure second-strike capability, Washington may be reluctant to employ Trident D5 missiles in all but the most extreme circumstances. Ultimately, both the air-launched cruise missile and the W76-2 warhead — neither of which would require any changes on on-Peninsula alliance posture — present useful options that may be worth consulting on more specifically in upcoming alliance dialogues if they can usefully demonstrate to Seoul that U.S. nuclear extended deterrence capabilities remain robust despite the ongoing shifts to North Korea’s posture.
‘San Francisco for Seoul’ to ‘Seoul for Seosan’
In the immediate hours after North Korea’s first flight-test of an intercontinental-range ballistic missile in 2017, analysts on both sides of the alliance began contending with a familiar alliance management problem from the Cold War: the “decoupling” problem. Just as Europeans — particularly the French — doubted that the United States would “be ready to trade New York for Paris,” so too would South Koreans and Japanese doubt whether the United States would, rhetorically speaking, trade San Francisco for Seoul, or Tampa for Tokyo. French President Charles De Gaulle wasn’t convinced, leading in part to France’s sustained pursuit of an independent nuclear deterrent as the Soviet Union produced ICBMs. With Yoon’s remarks and growing South Korean interest in nuclear weapons, history may not be repeating itself, but there may be something of a rhyme.
The “decoupling” problem hasn’t gone away since 2017 and is commonly voiced by proponents of an independent South Korean deterrent. Cold War analogies, however, as always, are inexact: The United States, in part, coped with the decoupling challenge by forward-deploying nuclear weapons to central Europe, seeking to offset NATO’s conventional inferiority to the Soviet Union’s quantitatively superior conventional strength. On the Korean Peninsula today, it is North Korea that is asymmetrically reliant on early nuclear weapons to use to offset its conventional inferiority. Nevertheless, North Korea continues to test and improve its intercontinental ballistic missiles. Pyongyang concluded its unprecedented year of missile testing in 2022 with a cliffhanger, testing a large-diameter solid propellant engine, suggesting that more responsive and survivable long-range missiles under Kim Jong Un’s control may not be too far off. The United States and South Korea are, in part, addressing and managing this problem in the course of their consultations through mechanisms like the reconvened Extended Deterrence Strategy and Consultative Group. The allies are additionally updating their war plans to acknowledge the nuclear dimensions of North Korea’s capabilities that can be brought to bear in a war.
North Korea’s development of tactical nuclear weapons, however, may put a new spin on how the allies — and South Korean military planners and leaders, in particular — think about the consequences of nuclear use by Pyongyang. In 2017, North Korea’s limited inventory of nuclear warheads, relatively imprecise array of nuclear-capable delivery systems, and poorly tested long-range missiles meant that the contours of nuclear use were somewhat constrained. Pyongyang would have resorted to initial nuclear use against regional targets that would enable a large-scale conventional attack on its territory — think ports, airfields, and command and control sites — to degrade the alliance’s ability to wage war. To encourage war termination on favorable terms to Kim and to dissuade further escalation by the United States, Pyongyang would have likely maintained its intercontinental range missiles in reserve, threatening their use only if the United States indicated that it would continue to wage war.
This basic strategy hasn’t changed for North Korea, but developments since 2017 have certainly fleshed out the credibility of what Pyongyang is threatening to do. The allies continue to cope with North Korea’s nuclear threats by threatening a regime-ending response, all while Kim threatens to “exponentially” expand the size of his nuclear warhead stockpile. The credibility of a regime-ending response hinges on the ability of the allies to comprehensively strike and disarm any North Korean nuclear systems that could be employed past the point of initial first use — all the more important now that North Korea has formally adopted a fail-deadly “dead hand” that’ll see all available nuclear weapons released in the event of Kim’s untimely death at the hands of South Korean or American ordnance. The growing survivability, diversity, and responsiveness of North Korea’s forces all but ensures that the allies could not, to a high degree of confidence, assure such a comprehensive strike would be successful and thus risk massive escalation after what may be a limited act of nuclear first-use by North Korea.
Where South Korea then might once have worried about whether the United States would trade “Seoul for San Francisco,” its decision-makers may now need to contend with whether they themselves would be ready to trade “Seoul for Seosan” (the latter being a conveniently alliterative South Korean airbase that hosts F-35As that may be the subject of a nuclear first strike). North Korea’s pursuit of tactical nuclear weapons and their potential use would force a terrible decision on any South Korean president: the prospect that choosing to proceed with a regime-ending campaign after North Korea’s initial nuclear use could lead to escalation against South Korean urban centers, including Seoul. The bar for a successful counterforce campaign remains high and it is more than likely that the totality of the two allies ‘conventional firepower, intelligence, surveillance, and reconnaissance capabilities, alongside America’s nuclear options, could not assure a sufficiently high probability of success in a disarming strike. The allies could, at best, limit some damage, but still suffer from North Korea’s nuclear use. Seeking assured invulnerability to North Korean nuclear attack is a mirage.
The bad news here is that Kim Jong Un may have more reason today than he’s ever had to believe that North Korean nuclear first use may not be a regime-ending event. Insofar as there is any “good” news, it’s that the severity of the decoupling problem may be attenuated by the incentives that now exist for Seoul and Washington begin thinking about managing and limiting, to the extent possible, nuclear escalation with North Korea — including past the point of North Korean nuclear first use. The alliance’s declared policy remains credible should North Korea massively escalate against a range of targets, including cities, in South Korea and Japan, but the introduction of tactical nuclear weapons may give Kim Jong Un greater belief that his nuclear use against military targets can precipitate early war termination by forcing the allies into a corner. Moreover, should North Korea massively employ tactical nuclear weapons against military targets, it is overwhelmingly likely that U.S. troops, their dependents, and other U.S. citizens residing in South Korea would perish, drawing the United States into the conflict, raising the prospect of nuclear exchange with North Korea. The most attractive solution to these problems is to avoid the prospect of a conventional crisis escalating to the point where North Korea could view nuclear weapons use as desirable to accomplish tactical ends on the battlefield (by preempting stealth fighters or other on-Peninsula strike assets). Importantly, neither U.S. nor South Korean nuclear weapons can solve this problem.
The ‘Nuclear Planning’ Boondoggle
Short of nuclear proliferation and U.S. nuclear weapons redeployment, a growing demand from many South Korean defense experts and strategists these days is that the United States open the door to Seoul’s participation in “nuclear planning” processes. After all, if NATO has a Nuclear Planning Group, why shouldn’t the U.S.-South Korea alliance? This sounds like a reasonable ask to many in Seoul: Some proponents of the idea see U.S. reluctance to discuss nuclear planning as evidence that South Korea is a junior ally compared to what’s perceived as a substantially more mature transatlantic alliance.
Washington’s reluctance has less to do with rank-ordering its allies and more to do with what’s meant by “nuclear planning.” What remains poorly understood — and somewhat poorly communicated by the United States — is that the NATO Nuclear Planning Group doesn’t really do what is traditionally understood to constitute “nuclear planning” despite its name. The group “provides a forum for consultation, collective decision-making, and political control over all aspects of NATO’s nuclear mission, including nuclear sharing,” per NATO. With the exception of nuclear sharing, which is bespoke to NATO and pre-dates the Non-Proliferation Treaty, the rest of what the Nuclear Planning Group does is largely addressed in existing consultative mechanisms between the United States and South Korea.
“Nuclear planning,” in the traditional sense, is more granular and sensitive than what even the Nuclear Planning Group does in the NATO context. This is largely what is sought today by South Korean officials and experts seeking to deepen the nature of consultations between Washington and Seoul. Planning, understood strictly, encompasses everything from targeting to operational considerations. In the United States, U.S. Strategic Command undertakes nuclear planning, which is carried out pursuant to the policy objectives and employment guidance set out by the president of the United States. These plans ensure that, in a crisis, a “menu” of nuclear options is available to the president of the United States. Allies do not participate in this process: Instead, at fora like the Nuclear Planning Group and the various bilateral consultative mechanisms that exist in the U.S.-South Korea and U.S.-Japan alliances, their views can be considered as part of the policy process that can inform nuclear planning by the U.S. military.
NATO’s nuclear sharing arrangement does mean that some nuclear planning, in the traditional sense, is carried out by parts of the alliance. This concerns the United States, the United Kingdom, and the five NATO states that host nuclear-certified dual-capable aircraft capable of delivering nuclear weapons. (France, despite being a nuclear-armed NATO state, does not participate in the Nuclear Planning Group and related NATO nuclear processes.) This component is handled by NATO military staff at Supreme Headquarters Allied Powers Europe and is sensitive to the political decisions on nuclear policy taken at the Nuclear Planning Group. This is partly a necessity of NATO’s nuclear mission: Without planning, the countries with dual-capable aircraft, in particular, would be unable to effectively carry out strike operations with nuclear assets should deterrence fail.
Many in Seoul and Washington might reasonably disagree about whether nuclear sharing or even planning would meet the alliance’s demands to deter North Korea. What needs to be clear, however, is that the United States is not unduly restricting Seoul from the types of nuclear policy consultations that are currently available to its NATO allies. To the contrary, NATO states may see something to desire in the relatively integrated nature of U.S. consultations on extended deterrence matters in East Asia compared to what might be interpreted as a somewhat dated, siloed process within the transatlantic alliance on nuclear matters, crystallized in the role played by organs like the Nuclear Planning Group. As far as extended deterrence consultations are concerned, the grass may be greener on the other side.
The Bottom Line: Political Solidarity and Trust
The most unfortunate short-term consequence of Yoon publicly mooting nuclear armament is what it does to trust and solidarity within the U.S.-South Korea alliance. These are ultimately the fundamental building blocks of any strong alliance, which relies on shared interests. Under the Trump administration, the United States took a sledgehammer to solidarity and trust by insisting on an extortionary approach to host nation support payments and unilaterally terminating military exercises without consultation. In addition to Seoul’s threat perceptions vis-à-vis North Korea, the experience under the Trump administration raised fundamental questions about the long-term reliability of the United States. Recognizing this, the Biden administration has sought to restore trust in the United States — not just in Seoul, but in allied capitals around the world.
After mooting the nuclear armament option, Yoon proceeded to note that “for now” the allies would proceed with various extended deterrence consultations. This phrase — “for now” — is familiar to any American who has recently discussed these matters in Seoul and speaks to how many pro-nuclear armament South Koreans are bargaining with the United States. Underlying this phraseology is the implication that Washington had better take Seoul’s demands seriously or else there’ll be no other option left but proliferation. In addition to bargaining with the United States, Yoon’s publicized comments have also made clear to pro-nuclear conservatives in South Korea that their preferred course of action is part of the policy decision-making space. Hard-bargaining, including through public signals, has risks — and not just with the United States.
North Korea, too, will take note of what this moment may imply about the resolve of the U.S.-South Korea alliance. Seoul’s increasingly public demands for new forms of nuclear reassurance from the United States implicitly reveal that South Korean policymakers bear doubts about what their conventional deterrent capabilities — including the three-axis system — can reasonably accomplish. A rational Kim can still be deterred through conventional means should Seoul and Washington continue to demonstrate that he cannot accomplish objectives averse to the alliance’s interests without suffering costs disproportionate to the benefits he might seek.
Imposing those costs does not require the effects of nuclear weapons, particularly given the North Korean leadership’s disregard for large-scale, unacceptable damage against military targets outside of the Kim regime’s survival itself. For better or worse, the regime’s survival, following massive nuclear use, can be threatened with conventional means alone and North Korean conventional aggression can be repelled by the superior capabilities of the U.S.-South Korea alliance. The Yoon administration’s deterrence messaging is veering off course from these fundamentals with a destabilizing focus on preemptive strikes, decapitation plans, and disproportionate retaliation. Most importantly, Seoul and Washington should rediscover the parallel pillar to deterring effectively: assurance. Just as Kim should understand the costs he might face, so too should he be assured that if he doesn’t transgress the alliance’s red lines, he won’t incur those costs. Incentivizing North Korean restraint and deterring escalation will require the alliance to leverage the full array of tools available, including those beyond military capabilities alone.
As the Biden and Yoon administrations continue to engage on extended deterrence, they should prioritize these essential matters to reduce the risk of nuclear escalation. To facilitate a unified and effective allied approach, the Biden administration should be clear — without precipitating a greater crisis in the alliance — that Seoul’s contemplation of nuclear proliferation as a bargaining tactic is ultimately unproductive and detrimental to allied security. Separately, the administration should privately communicate to Seoul that a South Korea that chooses to abrogate its non-proliferation commitments and pursue nuclear weapons could not be certain that U.S. extended deterrence assurances would persist. There remains little certainty about how successive U.S. administrations may view a nuclear-armed South Korea, which could be accommodated into a regional U.S. strategy to, for instance, counter China. Many in Seoul recall the precedent-setting U.S. civil nuclear agreement with India, which appeared to exhibit that non-proliferation principles could be subdued in pursuit of perceived geopolitical gain. But for now, the United States should not lose site of the essential role that non-proliferation has and continues to have for U.S. interests in Asia and elsewhere. The answers to improved allied security on the Korean Peninsula are unlikely to be found with nuclear weapons.
Ankit Panda is the Stanton Senior Fellow in the Nuclear Policy Program at the Carnegie Endowment for International Peace. He is the author of Kim Jong Un and the Bomb: Survival and Deterrence in North Korea (Oxford/Hurst, 2020). Follow him on Twitter at @nktpnd.
Image: The White House