Between Seoul and Sole Purpose: How the Biden Administration Could Assure South Korea and Adapt Nuclear Posture


Can Washington keep its friends feeling secure and, at the same time, reduce the prominence of nuclear weapons in its national security? That is the needle that President Joe Biden’s administration will try to thread as it faces growing policy tension between alliance management requirements and anticipated adjustments to the U.S. nuclear posture.

Regional threats in Asia and Europe are growing more complex, whether from North Korea’s burgeoning nuclear missile arsenal, China’s combative “wolf warrior” diplomacy, or Russia’s use of cyber attacks and attempted assassinations. The United States also confronts a credibility deficit resulting from President Donald Trump’s transactional treatment of America’s traditional security allies. As a result, U.S. allies in both Asia and Europe express greater fears of alliance decoupling and demand more U.S. assurances.

At the same time, growing fiscal pressures and a desire to revise some aspects of Trump’s nuclear policy propel putative changes to how the United States postures nuclear weapons, both to deter adversaries and assure allies. In particular, the Biden administration could seek to remove ambiguity about whether the United States would use nuclear weapons in response to non-nuclear attacks. The 2020 Democratic Party election platform avowed that “the sole purpose of [the U.S.] nuclear arsenal should be to deter — and, if necessary, retaliate against — a nuclear attack.” As vice president, Biden supported sole purpose, so at least the administration seems likely to consider it in the context of their policy reviews. A sole purpose declaration could give U.S. deterrent threats greater credibility and dampen crisis escalation risks with adversaries but potentially at some cost to alliance assurance.

Balancing risks across these policy streams will require deft diplomacy from the White House — not only with U.S. allies but also within the U.S. interagency process. There are real dangers that changes in deterrence posture could undercut assurance objectives, and vice versa.



In seeking to manage both policies, it will be crucial for U.S. officials to ground options in the complex domestic politics and security considerations of each of its alliance partners. Typically, the U.S. government speaks of “tailored” deterrence with respect to the threats adversaries pose. A more salient challenge now is “tailoring” assurance approaches to rebuild allies’ faith in the U.S. commitment to their security while avoiding potential commitment traps that could result in demands for ever more nuclear capability as symbols of assurance.

South Korea represents perhaps the tough case for U.S. balancing of assurance and nuclear posture imperatives. As the Biden administration develops alliance policies toward Seoul, and as it reviews U.S. deterrence capabilities and posture, a nuanced understanding of South Korean threat perception and policy orientation will be invaluable. Circumstances on the Korean Peninsula have evolved dramatically since the 2010 Obama Nuclear Posture Review, including the threat Pyongyang poses and a progressive government in Seoul focused on establishing inter-Korean peace. South Korean policymakers, therefore, may now be more concerned about the possible impact of U.S. nuclear posture changes on peace efforts with the North, for instance, than on the credibility of U.S. extended nuclear deterrence. Meanwhile, public opinion polls in South Korea show consistent majority support for the redeployment of U.S. tactical nuclear weapons, which were removed in 1991 as part of President George H.W. Bush’s Presidential Nuclear Initiatives.

This article explores how the alliance management challenge with South Korea intersects with putative U.S. nuclear posture adjustments and suggests some ideas on the types of initiatives the United States could implement to manage this tension. There is a narrow window of opportunity — approximately 15 months — between the start of Biden’s term and the next South Korean presidential election in spring 2022 during which Washington and Seoul can work the problem. Both capitals have a strong interest in avoiding potential politicization of nuclear issues. Ideally, U.S. policies should aim to forestall precipitous junctures in the alliance, most importantly a future South Korean request for tactical nuclear weapons redeployment.

South Korean Political and Security Context

The maturation of North Korea’s nuclear weapons and missile technology over the last decade is stunning. Among other achievements, it conducted four additional nuclear explosive tests beyond its initial tests in 2006 and 2009, including one assessed to be a thermonuclear design. It tested two (and paraded a third) long-range missiles that could target the continental United States. And it probably quintupled the amount of fissile material available for nuclear warheads, from an estimated stockpile of plutonium sufficient for five to seven warheads in 2010 to a combined stockpile of plutonium and highly enriched uranium sufficient for at least 30 warheads by 2020, and (according to a reported Defense Intelligence Agency assessment) perhaps up to 60.

In the context of North Korea’s nuclear advancements, since late 2017, South Korean President Moon Jae-in pressed forward with efforts to modernize South Korea’s defense and deterrence capabilities, build a new foundation for inter-Korean peace and stability, and foster new U.S.-North Korean nuclear negotiations. This approach attempts to sustain deterrence of the evolving threat from the North while also attempting to entice Pyongyang into a peace process that would provide the two Koreas more ownership of their future. (President Trump’s 2017 warnings of “fire and fury” against North Korea, and his threat to withdraw U.S. troops from South Korea unless Seoul paid more of the costs of hosting U.S. Forces Korea, presumably validated long-held desires among South Korean progressives, including Moon, for greater independence to conduct diplomacy with the North.) The crowning achievement of Moon’s peace efforts was the September 2018 Pyongyang summit declaration and associated agreement to implement military restraints and confidence building measures along the Demilitarized Zone. But, when U.S.-North Korean nuclear diplomacy stalled in early 2019, momentum toward a broader peace regime petered out.

South Korean threat perception regarding North Korea remains high despite warming inter-Korean ties from 2018 to 2019. Yet, South Koreans probably feel more threatened by North Korea’s arsenal of conventionally armed multiple rocket launch systems and short-range missiles than by its nuclear weapons and view Pyongyang’s nuclear developments more as a problem for the United States to solve than for Seoul. Increasingly, China looms larger as a regional threat following Beijing’s economic punishment of Seoul in 2017 for agreeing to host the U.S. deployment of Terminal High Altitude Area Defense missile defense systems. In 2019, public opinion polling, 75 percent of South Koreans offered a negative view of China, and 34 percent picked China as the top threat, followed by North Korea (31 percent) and Japan (28 percent).

Of course, North Korea’s ability to target nuclear weapons on U.S. cities fosters fears of alliance decoupling — that the United States would not sacrifice Los Angeles to defend Seoul, as the adage goes. Some conservative South Korean political and policy elites worry about the perceived efficacy of U.S. extended nuclear deterrence. They fret that changes in U.S. nuclear posture and declaratory policy might erode deterrence and/or embolden North Korean military adventurism. Deterrence concerns have been less prevalent among South Korean progressives, who instead tend to seek greater autonomy within the alliance and blame Washington for standing in the way of improved inter-Korean relations. Some prominent officials and scholars in Seoul have even begun to talk more openly about life after the U.S.-South Korean alliance. That said, there seems to be a broadly shared view in Seoul that the alliance approach to deterrence and defense cooperation needs updating. These sentiments transcend and predate Trump’s disdainful view of alliances and his efforts to extort higher burden sharing payments from Seoul.

Beginning in 2017, South Korean conservative and centrist political parties adopted election platforms calling on the United States to redeploy tactical nuclear weapons on the Korean Peninsula and for unspecified “nuclear sharing” arrangements similar to NATO. Proponents of reintroducing such weapons argue they would counter potential North Korean nuclear coercion, repair perceived damage to the U.S. extended nuclear deterrence umbrella over South Korea, and create more leverage to induce North Korea to disarm. Some individual politicians threatened that, if the United States does not re-station nuclear weapons, South Korea would have no choice but to develop its own nuclear arsenal. These views tend to be dismissed in Washington, with a strong presumption that they are more emotional than strategic. Notably, the ruling Democratic Party has not staked out a position on tactical nuclear weapons and actively discourages discussion of nuclear weapons issues among its members, though some progressive scholars also warn about future proliferation. There are clear signs of increasing frustration among South Korean politicians and security professionals regarding alliance consultations on deterrence that speak to a U.S. credibility deficit. Calls for tactical nuclear weapons or an independent nuclear arsenal probably reflect real anxieties, even if they take the form of populist national security appeals from right-wing politicians.

Regular public opinion polling suggests that a majority of South Koreans supports acquiring nuclear weapons. Public opinion on nuclear issues in South Korea tends to be uninformed, however, and, if the potential adverse economic and security implications of nuclear weapons development were clear, support could decline significantly. (Eye-catching polls showing 60 percent support for nuclear weapons tend not to differentiate between U.S. redeployment of tactical nuclear weapons and South Korean development of an independent nuclear arsenal.) There would also be very strong “not in my backyard” opposition from local communities around hypothetical nuclear weapon storage and deployment sites. It is plausible that the South Korean government, were it to fully examine the issue, might conclude the political, financial, and bureaucratic costs of preparing for tactical nuclear weapons redeployment are too high. Even so, some South Korean politicians have sought to mobilize public support for nuclear weapons — unsuccessfully thus far — and, under the right circumstances, it could prove a disruptive political force for the alliance or even a self-fulfilling prophecy.

The consequence of changes in South Korean domestic politics and threat perception is a growing alliance assurance and commitment credibility problem for the United States. There are ample indicators of plausible future developments that would force Washington to make difficult choices between alliance management, nuclear posture, and nonproliferation policies. Twice in the recent past, for example, senior South Korean officials have raised possible redeployment of tactical nuclear weapons with U.S. counterparts. In October 2016, South Korea’s then-deputy national security advisor reportedly requested the administration of Barack Obama to re-station tactical nuclear weapons. And, in September 2017, then-Defense Minister Song Young-moo reportedly asked U.S. Secretary of Defense James Mattis to consider the same. The latter incident is notable given Song was serving under the progressive Moon administration, and the Blue House was forced to walk back his remarks. Washington would be wise to consider the sentiment behind these requests and ways to rebuild alliance credibility that dampen such reflexive interest in nuclear weapons redeployment.

South Korean Deterrence Posture

Given perceived threats from North Korea and China, and concerns about the future of the U.S.-South Korean alliance, there appears to be broad political and expert agreement in Seoul regarding the need for stronger independent military capabilities for a variety of missions, as well as greater resiliency and survivability of conventional deterrent platforms. This view is laudable to the extent it provides a strong foundation for re-building alliance credibility, even if it also indicates possible security hedging against future U.S. retrenchment from Asia.

South Korea’s defense budget had been steadily increasing under the conservative Lee Myung-bak and Park Geun-hye administrations, but, under Moon, its rate of increase accelerated from a 4 percent jump in 2017 to 8.2 percent in 2019. Partly, these increases reflect force readiness expenditure and procurement of advanced weapons platforms such as the F-35. But the Moon administration also quietly carried forward several of the North Korean-specific deterrence and denial programs prior conservative administrations launched. These include production of ballistic and cruise missiles of increasing range, payload, and accuracy that are part of the “kill-chain” and “Korea massive punishment and retaliation” plans.

Since South Korea launched and then abandoned under U.S. pressure a secret nuclear weapons program in the 1970s, Washington has sought to retard and/or manage South Korea’s fuel cycle and ballistic missile capabilities. These constraints include withholding permission to reprocess or enrich U.S.-flagged nuclear material and limiting the payload and range of South Korean missiles. Unlike Japan, South Korea does not have a ready stock of fissile material, but it does have robust missile delivery options. Some South Korean experts estimate it would take 18 months to develop and produce a bomb.

The Moon administration also advanced a program to acquire a nuclear-powered attack submarine. The rationale for a nuclear-powered attack submarine over existing diesel-electric submarines is to provide better anti-submarine warfare, sea denial, and counter-weapons of mass destruction capability against a possible North Korean sea-based nuclear weapons platform. With a capability for vertical launch of ballistic missiles, however, it could also provide a potentially more survivable platform for punitive offensive strike options. In August 2020, the South Korean National Assembly approved a defense budget with funding to build a nuclear-powered attack submarine, although significant design and procurement issues loom. Among these hurdles, South Korea does not currently have a uranium enrichment capability to produce naval reactor fuel. South Korea’s Deputy National Security Advisor Kim Hyun-jong reportedly requested provision of enriched uranium fuel during a September 2020 visit to Washington, thus creating a complicated linkage between deterrence and nuclear energy cooperation with the United States. It remains unclear whether, and at what pace, the submarine program will move forward, with or without U.S. involvement.

South Korean Views on Sole Purpose and U.S. Nuclear Deterrence

During the 2010 U.S. nuclear posture review process, officials in the conservative Lee administration (as well as defense scholars at official South Korean research institutes) raised concerns about putative changes in U.S. nuclear posture then under consideration. Unlike some Japanese counterparts, who were specifically concerned about the U.S. decision to retire the Tomahawk nuclear-armed cruise missile, South Korean experts seemed more concerned about a possible shift to embrace a “sole purpose” declaration. There is a prevailing belief among many defense experts in South Korea that the “nuclear” aspect of U.S. extended deterrence to Seoul is critical and that efforts to reduce the salience of nuclear weapons headlined by a shift in declaratory policy would be detrimental to Seoul’s security. According to this view, North Korea (or China, for that matter) might perceive that sole purpose diminishes credibility or extended deterrence and increases the feasibility of using nuclear coercion to decouple the alliance. Such perceptions persist, even though there are hardly any imaginable scenarios in which Washington would conclude that first use of nuclear weapons was preferred to conventional alternatives.

During the Obama administration, Washington took South Korean concerns about shifts in nuclear policy on board by stipulating in its 2010 Nuclear Posture Review that the United States would not adopt a universal policy of sole purpose but instead would “work to establish the conditions under which such a policy could be safely adopted,” and by convening regular extended deterrence dialogues. These dialogues featured a range of activity, including efforts to improve South Korean working-level understanding of U.S. deterrence operations (through briefings and base visits, for instance), as well as high-level statements of solidarity for strategic messaging aimed at political elites and the South Korean public. Washington probably saw these dialogues mainly as a way to satisfy South Korean desires for more assurance while avoiding major changes to the alliance. Accordingly, Washington maintained implicit red lines on the content, the overstepping of which might feed South Korean interests or perceptions about a Republic of Korea role in U.S. nuclear employment policy or nuclear sharing. (Anecdotal comments from some South Korean participants indicate that these dialogues did not fully address their interest in understanding when and how U.S. decisions about potential employment of nuclear weapons on the Korean Peninsula would be made.)

The context around South Korean concerns about sole purpose has shifted considerably since 2010, as has Korean politics from the conservative Lee administration to the progressive Moon administration. The 2018 U.S. Nuclear Posture Review — which argued for augmented nuclear capability to address perceived deterrence gaps and for more ambiguity about the potential for a nuclear response to a “non-nuclear strategic attack” — raised concerns in South Korea about a perceived lowering of the threshold for nuclear use and the attendant implications for crisis escalation with North Korea. South Korean scholarship does not indicate whether these concerns were offset by any assessed gains for the credibility of U.S. extended nuclear deterrence. Meanwhile, under a progressive presidential administration, South Korea’s defense community in general has lowered the volume on its nuclear rhetoric, so there has been less focus in seminars and commentary on U.S. nuclear policy except as it relates to North Korea. Post-Trump and “fire and fury,” there is also a latent fear about being dragged into a conflict with North Korea or China by a reckless American president. The South Korean reaction to North Korean contingency planning anecdotes from Bob Woodward’s Rage — namely the public denial that any U.S.-South Korean military operations plans involve the use of nuclear weapons — is indicative of this concern. It is difficult to say how much of the perceived credibility deficit is due specifically to Trump’s alliance approach — concerns that should now fade for the time being — versus broader changes in the security environment.

So how might U.S. consideration of a sole purpose declaration be received in Seoul today? There has been no recent South Korean scholarship on this issue, so the following is mostly conjecture based on informal conversations with South Korean colleagues. Potentially, the Moon administration might welcome a U.S. sole purpose declaration as helping to facilitate the eventual denuclearization of the Korean Peninsula by diminishing the prominence of the nuclear threat in the U.S. “hostile policy” toward North Korea. Underneath such political acceptance, defense officials are likely to question what steps Washington is planning to ensure that extended deterrence through the alliance would remain both credible and sufficiently nuclear. In this context, tactical nuclear weapons redeployment and nuclear sharing interests are likely to surface anew. Further, conservatives could attempt to mobilize alliance decoupling concerns with a view toward the 2022 presidential contest, in which they are likely to criticize Moon’s inter-Korean peace policy and reiterate arguments in favor of tactical nuclear weapons and/or a South Korean nuclear capability. To the extent the South Korean public seizes on concerns about the credibility of the alliance, it is possible that tactical nuclear weapons may be seen as the solution — a development Washington would want to avoid.

U.S. Policy Considerations

The merits and demerits of moving U.S. declaratory policy toward sole purpose or, further, to no-first-use, have been debated extensively elsewhere. Suffice to say here that, in principle, a U.S. sole purpose declaratory policy would not be inconsistent with extended nuclear deterrence, nor with the redeployment of tactical nuclear weapons in South Korea and the continued presence of U.S. tactical nuclear weapons in Europe through NATO’s nuclear sharing arrangement. The general rationale for forward deployment of nuclear weapons — to ensure that nuclear weapons cast a shadow over regional contingencies — is linked but also separable from issues of calculated ambiguity about the circumstances and timing of a potential decision to employ nuclear forces. Some analysts might argue precisely that forward deploying U.S. nuclear weapons in Asia could redress misperceptions among U.S. adversaries that sole purpose would somehow signal diminished political credibility of U.S. extended deterrence commitments. (Indeed, some Americans — including several U.S. members of Congress — have supported returning tactical nuclear weapons to South Korea to counter North Korea’s nuclear missile advances.) Others would argue that a sole purpose declaration would actually strengthen the credibility of deterrence signaling to adversaries and allies alike, thus diminishing the perceived need for additional forward deployments that could be destabilizing.

If the Biden administration opts to adjust U.S. declaratory policy, it may face requests from Seoul (and perhaps Tokyo) for countervailing adjustments to nuclear capability deployed in East Asia, including possibly forward deployment. There are manifest operational and tactical challenges with forward deploying tactical nuclear weapons. Beyond these, potential deterrence or alliance assurance gains likely would be outweighed by wide-ranging political, normative, and economic risks and costs, except under extreme circumstances. Many of these effects could, in fact, attenuate the likely benefits of shifting to sole purpose.

The potential immediate and second-order effects of a U.S. decision to reintroduce tactical nuclear weapons to the region extend to several adjacent policy jurisdictions and a host of domestic and foreign policy interests, including:

U.S.-Chinese Competition

U.S. forward deployed tactical nuclear weapons could provoke a deepening arms race with China whose second order consequences could worsen the U.S. ability to project power in the region. U.S. policy choices to bolster regional containment of more aggressive Chinese operations in East Asia, while mitigating arms racing and conflict escalation risks, could complicate or narrow options for addressing South Korea’s deterrence concerns, and vice versa. South Korean officials have expressed trepidation about joining any security initiatives — such as the so-called Quad framework involving the United States, Japan, Australia, and India — that appear to be targeted at China, and instead have sought to maintain positive relations with Beijing given the economic and trade incentives involved.

Nuclear Policy Toward North Korea

North Korea could respond by deploying its own battlefield nuclear weapons (a possibility Kim Jong-un touted in his address to the January 2021 Eighth Party Congress), dramatically increasing the risks of nuclear use in even limited conflict scenarios between the two Koreas. It is also plausible that, at some point, Washington would again pursue negotiations to reduce the threat Pyongyang’s burgeoning nuclear arsenal poses. Putative tradeoffs in such negotiations could impact U.S. military and/or extended deterrence operations with South Korea and, thus, options for strengthening deterrence. Redeploying tactical nuclear weapons could stymie negotiations.

U.S.-Japanese Alliance

Tokyo will keep a close eye on U.S.-South Korean deterrence discussions amidst growing fears of Chinese encroachment and deliberate Japanese progress to develop conventional missile strike options. Trilateral coordination remains beset by lingering South Korean-Japanese legal-political disputes over territory, history, and reparations for Japan’s prior occupation of the Korean Peninsula. U.S. forward deployment of tactical nuclear weapons could also deepen Japanese domestic political challenges, provoking opposition from civil society organizations that strongly favor Japan joining the Treaty on the Prohibition of Nuclear Weapons.

Arms Control, Nonproliferation, and Disarmament

U.S. efforts to promote nonproliferation and reduce the salience of nuclear weapons globally likely would be set back. Russia, China, and others would use tactical nuclear weapons deployment as a wedge issue to frustrate U.S. policy in multilateral venues (in the Nuclear Non-Proliferation Treaty review process, for instance). It would also provide China a convenient excuse to eschew discussion on future arms control arrangements.

Budget Controls

The logistics and security architecture for the forward deployment of tactical nuclear weapons do not currently exist and would be very costly to build. Furthermore, unless they were shifted from Europe or withdrawn from reserve storage (leaving it at a reduced level), the United States would need to build additional B-61s (the only plausible weapon for a forward deployment) for a forward deployment in Asia. At a time when budget pressures on U.S. nuclear modernization are mounting, funding for new forward deployment programs presumably would impinge on other priorities.

As the Biden administration sorts through these linkages — and considering the general ascendancy of regional policy perspectives over functional ones in U.S. foreign policymaking — it would probably arrive at a hierarchy of preferences something like the following: 1) sustaining the U.S.-South Korean alliance; 2) preventing any political or technical momentum toward an independent Republic of Korea nuclear arsenal; 3) avoiding actions that would provoke North Korean or Chinese responses and complicate threat reduction negotiations with either or both; and 4) averting further erosion of nonproliferation norms and technical barriers to nuclear weapon acquisition that would attend green-lighting South Korea’s development of nuclear fuel cycle capabilities.

In weighing alternate policy outcomes, it is difficult to imagine the conditions under which Washington would conclude that the costs and risks of forward deploying tactical nuclear weapons to the region, including redeployment to Korea, would be less than the likely alliance credibility benefits. Some analysts might argue that threatening to forward deploy tactical nuclear weapons could bolster leverage over North Korea and China, and that a U.S. deployment would not necessarily provoke counter-deployments. Yet, recent North Korean and Chinese behavior suggests either could opt to deploy their own tactical weapons or other offsetting capabilities that would compound escalation risks more than U.S. forward deployment would enhance U.S. and allied security. (China could also again use economic coercion to punish South Korea, as it did in 2017.) Accordingly, these preferences probably put a premium on steps to consolidate alliance deterrence capabilities in ways that do not involve deploying more nuclear weapons to the region or increasing overt nuclear signaling. This approach could coincidentally help to avoid a potential trap in South Korean politics by which tactical nuclear weapons redeployment becomes the preferred solution to a perceived alliance credibility deficit.

U.S. Policy Objectives and Options

Regardless of whether the Biden administration opts to change its nuclear declaratory policy, for the duration of the Moon administration, it would make sense for Washington to pursue initiatives aimed at rebuilding public and elite trust in the alliance among the South Korean body politic. It seems axiomatic that an ally could never be 100 percent assured. Therefore, the objective of U.S. policy would be to repair damaged U.S. political credibility to a level sufficient to provide confidence in Seoul that changes in U.S. nuclear posture would not diminish either its security or the efficacy of extended nuclear deterrence.

Alliance assurance work could proceed along two axes: strategic messaging and innovating functional deterrence capacity against evolving threats. The U.S. and South Korean expert communities have done considerable thinking on these issues, focusing, among other areas, on strengthening conventional deterrence, transforming the alliance, and restoring strategic competence. Several types of initiatives could follow. For example, the two governments could launch a reinvigorated strategic dialogue at the ministerial level — perhaps joined also by national security advisors — to address questions of commitment. Such dialogue could involve more explicit political engagement, including with members of the South Korean National Assembly and U.S. Congress. Reconfigured dialogues on regional security and deterrence among governmental experts and military officials — also including staff from parliamentary national security committees — would be a useful complement to ministerial-level meetings.

Dialogues (and associated strategic messaging) would need to result in practical changes to the alliance. With major shifts in the basing of U.S. Forces Korea, coordination and communication challenges have grown. This creates both a need and an opportunity to recapitalize alliance capabilities and structures to better take advantage of South Korea’s growing independent defense and deterrence competences. For example, the two might prioritize augmenting conventional deterrence offsets — capabilities, readiness, exercises, and manpower — as well as building more autonomy and robustness into Republic of Korea asymmetric warfare and counter-weapons of mass destruction capabilities. These considerations would also have to factor into the politically fraught discussions on the timing of, and conditions for, transferring war-time operational control authority from U.S. Forces Korea to South Korea.

Revamped contingency planning is a third type of valuable initiative. It is especially important given the growing danger of crisis escalation on the Peninsula. Developing shared understanding among political and military leaders about options for crisis signaling and provocation response is useful both for building alliance political cohesion and for identifying and managing potential sources of disunity between Seoul and Washington. Shifting away from the past playbooks that relied heavily on nuclear signaling practices, such as nuclear bomber overflights, and instead emphasizing robust conventional deterrence should be a central feature of responses to future North Korean provocations.

Notably, some U.S. experts argue explicitly that, in light of North Korea’s nuclear arsenal, the U.S.-South Korean alliance needs to lean more on extended nuclear deterrence, not less. “NATO-like” initiatives would be one means of elevating the role of nuclear weapons. Such initiatives could include standing up deliberative structures to build political consensus about the role of nuclear weapons or taking preparatory steps for a South Korean dual-capable aircraft program. Some analysts even go so far to recommend permitting “friendly proliferation” of U.S. nuclear weapons.

The central issue with invoking the NATO analogy, of course, is the applicability and desirability of replicating the nuclear sharing pillar of the transatlantic alliance. Could a dialogue advertised as “NATO-like” avoid defaulting to issues of re-stationing tactical nuclear weapons, or at least creating the perception or expectation of such a path? Perhaps, but, if the U.S. government has no intention or desire to end up with a nuclear sharing arrangement with South Korea, or to forward deploy tactical nuclear weapons on the Korean Peninsula — except in extremis — there is considerable risk in starting down any road that looks or feels “NATO-like.” Indeed, there could be better value in a direct, public statement by a senior U.S. official stipulating that, though nuclear forward deployment is a nonstarter, Washington remains committed to sustaining effective deterrence through the alliance. Even if making NATO references might yield political mileage with South Korean conservatives, it is a potentially slippery slope that could ultimately end up feeding South Korean beliefs that more nuclear weapons — perhaps even an independent nuclear weapons capability — are necessary for the country’s defense. Given the challenging policy context described above, forestalling or, if need be, deterring such beliefs would better serve U.S. interests.

The Bigger Picture

If the Biden administration opts to adjust U.S. nuclear posture, including declaratory policy, it will no doubt do so in consultation with U.S. allies. In any case, as Washington invests in repairing alliances from the damages of the Trump era, it could address concerns about nuclear posture through various types of assurance initiatives like those discussed above.

For the U.S.-South Korean alliance, the spring 2022 South Korean presidential election will mark an important point for evaluating the results of efforts to build stronger credibility and commitment within the alliance. That election will most likely be a referendum on Moon’s handling of the South Korean economy amidst the COVID-19 pandemic. North Korea and/or U.S. alliance issues are unlikely to be major factors, barring a military crisis. But the election results could have important impacts on U.S. efforts to manage the tension between alliance assurance efforts and nuclear posture adjustments.

Victory by a progressive candidate — which seems more likely based on progressives’ strength of performance in recent elections and continuing disarray among conservative parties after the 2017 impeachment of President Park Geun-hye — would probably result in the continuity of many of the policies the Moon administration advanced. The potential for the alliance to suffer a crisis of political credibility seems relatively low, although there could be plenty of disagreement over other issues, not least probable disjuncture between Seoul’s inter-Korean peace push and Washington’s default pressure policy toward North Korea. A conservative candidate victory, on the other hand, could increase the likelihood that the new administration in Seoul would follow through on election platform promises to request U.S. redeployment of tactical nuclear weapons, notwithstanding the results of efforts to strengthen the alliance. The format of such a request (private versus public) and contingent terms (allusions to an independent arsenal if Washington refuses) would be important factors in shaping U.S. response options. In such circumstances, it would be very difficult for U.S. intelligence analysts and political leaders to accurately assess South Korean intentions: Would a request for tactical nuclear weapons essentially be a South Korean bargaining ploy, or merely a pretense to justify an independent weapons capability if Washington refused?

The potential for a future South Korean nuclear weapons program is not new, though the stresses on U.S. security alliances as a primary means of preventing future proliferation are greater now. After four years of Trump’s alliance policies, and in light of deep American political polarization, there are undeniable and growing incentives for U.S. allies to begin to hedge against a perceived decline in American credibility to carry through on security commitments. Allied hedging could drive efforts to strengthen defense capabilities independent of the United States, which may intrinsically help ease alliance anxieties. Yet, it could also cause U.S. allies to demand ever more nuclear signals of assurance in ways that complicate U.S. efforts to deter adversaries while avoiding conflict commitment traps or aggravating regional security dilemmas. South Korea is an important test case for the Biden administration to strike this balance. It is unlikely to be the only, or the last, such case for Washington to manage.



Toby Dalton is a senior fellow and co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace. This essay was commissioned by the American Nuclear Policy Initiative (ANPI). The author is grateful for feedback from participants in a January 12, 2021 ANPI seminar, from colleagues in Washington and Seoul, and from two anonymous reviewers.

Image: U.S. Air Force (Photo by Airman 1st Class Dillian Bamman)