Speaking Loudly and Carrying a Small Stick: South Korea Backtracks Over the Nuclear Rubicon

Earlier this year, South Korean President Yoon Suk-yeol made a pronouncement that no leader in Seoul has ever made before. He stated that South Korea—which benefits from US extended nuclear deterrence—could consider acquiring its own nuclear weapons if “North Korean provocations continued intensifying.” This claim attracted international attention, particularly in Washington, Pyongyang, Beijing and Tokyo.

Yoon’s statement was opaque, starting with the conditional nature of the pronouncement—how much more “intense” would North Korea’s threat level have to become before triggering South Korean nuclear weapon development? Moreover, Yoon added that the Republic of Korea (ROK or South Korea) could become a latent/threshold nuclear weapon power with an industrial and scientific capacity to produce nuclear weapons on a short time scale. How quickly that could actually be accomplished is unclear, however, given the country’s current dependence on foreign sources of nuclear material even for its civil nuclear energy program.

Given the controversial nature of these statements, Yoon’s presidential office later backtracked, offering the “clarification” that Yoon was simply expressing his “firm commitment to defending the nation” against North Korea’s nuclear threats, and while the “worst case scenario must be taken into consideration,” “the principle of abiding by the [Nuclear] Nonproliferation Treaty holds.”

Washington took notice of its worried alliance partner, starting with a clear step-up in demonstration of commitment to extended nuclear deterrence for South Korea—including a visit by Secretary of Defense Lloyd Austin and combined US-ROK air warfare training featuring F-35s, F-22s, B-52s and B1Bs (this is in addition to regular combined military exercises, and naval exercises featuring US aircraft carrier strike groups). Washington also agreed to more bilateral consultation with Seoul regarding the US nuclear umbrella, going beyond the revival of the Extended Deterrence Strategy and Consultation Group (EDSCG). Additionally, the Biden administration agreed to the instituting of joint tabletop exercises that involve responses to North Korean nuclear weapons use and approached South Korea and Japan about forming a trilateral consultative mechanism on extended nuclear deterrence.

Nonetheless, elite conservative South Korean politicians have continued to argue that Seoul should consider developing nuclear weapons. In late January, Hong Joon-pyo, a veteran heavyweight in the conservative People Power Party (PPP) and current poll leader to be the conservative nominee for the next presidential election, indicated support for President Yoon’s call for South Korea to develop nuclear weapons under certain conditions. Also in January, Seoul Mayor Oh Se-hoon, a star conservative politician and another favorite for the next presidential race, argued for South Korean nuclear weapons in mid-January. Oh then doubled down on that stance in a high-profile March interview with Reuters in which he called for South Korean nuclear weapons even in the face of costs and risks from international opprobrium (sanctions, strained diplomatic ties, etc.). A national assemblyman and former chairperson of the PPP, Chung Jin-suk, broached South Korean indigenous nuclear weapons in late February. North Korean defector and current South Korean National Assemblyman Thae Yong-ho has been on the record multiple times calling for South Korean nuclear weapons. These instances demonstrate how this debate, which was long confined to fringe views and voices, has increasingly become mainstream discourse in South Korea.

All this raises the question: How serious is the risk that South Korea acquires nuclear weapons or develops a latent/threshold nuclear capacity? The short answer to this question is “not very”—at least in the current environment, the international political cost of indigenous nuclear weapons for South Korea is too high. But behind the debate in South Korea is a complex problem of nuclear deterrence and ally reassurance on the Korean Peninsula, one which will loom in the background of the US-ROK military alliance for quite some time.


Impoverished, dysfunctional North Korea has nuclear weapons, no apparent intention of surrendering them, and a habit of threatening South Korea. Wealthy, high-functioning South Korea has no nuclear weapons but does have a powerful conventional military force, an alliance with the US, and the protection of US extended nuclear deterrence. From Seoul’s perspective, this arrangement has been acceptable as long as Pyongyang’s nuclear arsenal was minimal in quantity, limited in quality and likely unreliable—that is, not a real risk to South Korea. That situation has changed in recent years, however, and with it, South Korea’s willingness to rely on US extended nuclear deterrence in its current form. In a nutshell, the ROK’s concerns about the US nuclear umbrella require the arrangement to evolve; failing to seriously address the situation creates a potential path to latent/threshold nuclear capability or even actual construction of indigenous nuclear weapons.

The crux of South Korea’s worry arises from the fact that North Korea’s nuclear arsenal is growing quantitatively and improving qualitatively. Although North Korea has in some form possessed nuclear weapons for nearly twenty years, Pyongyang’s nuclear arsenal was extremely rudimentary for most of that period. Over time, however, the North’s nuclear arsenal graduated to a minimal nuclear deterrent, including intercontinental ballistic missiles that can range the United States, and is now likely on track to achieve the status of real second-strike capability.[1] This forces South Korea to consider that North Korea might not conclude that the US president (the sole possessor of launch authority for US nuclear weapons) would be willing to sacrifice (e.g.) San Francisco for Busan, were deterrence to fail.[2] The uncertainty of this trade provokes doubt among some South Koreans regarding the credibility of the US’s extended nuclear deterrence for South Korea. This problem is only set to worsen as Pyongyang develops tactical nuclear weapons that could theoretically be employed in such a way as to lead the Kim Jong Un regime to believe that it could strike South Korean targets with nuclear weapons without nuclear retaliation by the US.

North Korea’s improved and increasing nuclear capabilities have been accompanied by changes to its nuclear posture, which now seems to include possible nuclear “first-use” (presumably under conditions of existential threat to national/regime security) and a version of a nuclear “dead-hand” (“automatic” nuclear weapons launch if regime leadership and/or command-and-control is destroyed). The evolution in Pyongyang’s nuclear capability and accompanying posture creates the foundation for a far more aggressive overall coercive strategy vis-à-vis South Korea, up to and including trying to de-couple the US-ROK alliance and gain a decisive political/geopolitical/diplomatic advantage over Seoul on the Korean Peninsula.

Is an Ounce of Reassurance Worth a Pound of Deterrence?

As argued above, despite the growing North Korean nuclear menace vis-à-vis South Korea, Seoul does not intend to develop indigenous nuclear weapons in the short-/mid-term future, the trial balloons by President Yoon and other conservative elites notwithstanding. South Korean nuclear weapons acquisition would be too risky and costly: potential consequences would likely include heavy sanctions, diplomatic isolation and suspended access to civil nuclear materials in the international market (this would be a major blow to South Korea’s domestic nuclear power industry and nuclear power station exports). Why, then, have Yoon and other South Korean political elites broached the possibility of South Korea going nuclear?

The prevailing interpretation is that a combination of factors is at play. One factor is that the Yoon administration wanted to ratchet up pressure on Washington to take more seriously South Korean concerns about the credibility of extended nuclear deterrence, thus accelerating (even eliciting) improved policies from the US. A second factor is that the administration wanted to signal to North Korea and China that Pyongyang’s growing nuclear arsenal could result in an unwanted (from their perspective) regional security development, absent North Korean restraint and willingness to negotiate about denuclearization. A third factor is related to domestic politics. Conservative political elites—President Yoon, Mayor Oh, party heavyweight Hong, and party leader Chung—were responding to perceived popular dissatisfaction about US extended nuclear deterrence, notably taking a cue from numerous opinion polls showing that 65-70 percent of South Koreans favor development of independent nuclear weapons.

Defense and security elites in Seoul and Washington have responded with a two-pronged approach, aiming at both augmenting deterrence of North Korea and increasing reassurance for South Korea that US extended nuclear deterrence vis-à-vis North Korea is credible. One prong is that the US has agreed to more frequent deployment and exercise of “strategic” assets to and around the Korean Peninsula. Serving as both a deterrent message to Pyongyang and a reassurance message to Seoul, these more frequent US-ROK joint military deployments and exercises have also been made more visible through assertive official messaging. Fundamentally, however, such measures are just an indicator of US nuclear capability, about which there already was no doubt in either Pyongyang or Seoul.

The second prong has been to promise greater consultation for South Korea regarding US extended nuclear deterrence. Going beyond the Extended Deterrence Strategy and Consultation Group revived in 2022, the 2023 Biden-Yoon summit promulgated the Washington Declaration, which has introduced the US-ROK nuclear consultative group (NCG) dedicated to institutionalizing South Korea’s larger role in shaping extended nuclear deterrence for the Korean Peninsula. Inter alia, the US and South Korea have agreed to carry out tabletop exercises (already in February 2023, with a follow-on in May and more going forward), including scenarios involving North Korean nuclear weapon use. The idea is to improve South Korean opportunities to understand US policy, posture and logic regarding nuclear use in an extended deterrence context, and on that basis, to communicate Seoul’s position. This may (or may not) function as a first set of steps toward greater integration of South Korea into US extended nuclear deterrence planning (i.e., mutatis mutandis, a version of NATO’s Nuclear Planning Group). It will, at least, likely fill in gaps of South Korean elite understanding about how the US makes and executes decisions to use nuclear weapons on behalf of allies. That said, nothing can truly dispel the underlying fear of South Korean political elites that the US president might lack the will (both personal and political) to strike North Korea with a retaliatory attack on behalf of South Korea if it meant potentially trading San Francisco for Busan.

Ironically, as a South Korean elite justification and motivation for broaching an indigenous nuclear deterrent, the concern that the US president might not push the nuclear button on behalf of South Korea is perhaps a misreading of the anxieties of South Koreans regarding the US as an alliance partner extending nuclear deterrence. Indeed, there is a conundrum in much of the polling data asking the general South Korean population about its desire for South Korean nuclear weapons: To wit, there is significant overlap between the part of the population that trusts (and approves of) the US as an alliance partner and the part that claims to want South Korean nuclear weapons. For example, a poll conducted by the Chicago Council of International Affairs/Carnegie Endowment for International Peace shows that for every incremental increase in the respondents’ belief that the US would defend South Korea in case of conflict with the North, there is an increase in support for independent South Korean nuclear weapons. Those with low confidence in the US defending South Korea responded 56 percent positively to the question of indigenous South Korean nuclear weapons. Those with very high confidence in the US defending South Korea responded 77 percent positively to the question of indigenous South Korean nuclear weapons. A 2020 study in the Journal of Conflict Resolution reaches a similar result on the basis of surveys, as high belief in the credibility of US extended nuclear deterrence correlated with higher desire for independent South Korean nuclear weapons.

The data from these studies undermine the notion that the driving force behind a general South Korean popular desire for nuclear weapons is potential US fecklessness in the face of the rising North Korean nuclear threat vis-à-vis South Korea. How can one account for this conclusion? A likely possibility is that the South Korean public actually fears undesired employment of nuclear weapons by the US in case of hot war on the Korean Peninsula.[3] For example, a 2022 article in the Journal of Conflict Resolution reported results of a scenario-based study with 2,000+ South Koreans in which only 27 percent of study participants desired US nuclear retaliation against North Korea, even if North Korea attacked Busan with a nuclear weapon. When the author presented this figure in a closed-door seminar on extended deterrence on the Korean Peninsula, senior US military officers were dumbfounded at the low number, routinely estimating the number to be 50-80 percent, and thus neatly encapsulating the problem of the potential mismatch between what South Koreans and the US might consider to be the best course of action against North Korea, should nuclear deterrence on the Korean Peninsula fail.

Roosevelt Redux

Thus, it seems the problem with South Korean nuclear proliferation might rest on the possibility that different stakeholders want nuclear weapons for different—indeed counterposed—reasons. If, as suggested by the foregoing, US extended nuclear deterrence credibility is not the issue for the South Korean general public, but is the concern for South Korean elites, then the US is in a bind. On the one hand, Washington’s efforts to dissuade the desire of the South Korean public for nuclear weapons risk disappointing South Korean elites who fear degraded US extended deterrence credibility, thus inciting them to consider developing nuclear weapons to fill the perceived credibility gap. On the other hand, aggressive attempts to reassure South Korean elite belief in US credibility might raise popular alarm that the US is potentially inclined to use nuclear weapons on behalf of South Korea when it would be undesirable, thus incentivizing the Korean public at large to support indigenous nuclear weapons in order to ensure sovereignty of nuclear weapon use. In either case, one South Korean group could read the actions of the US as entailing a need for Seoul to build its own nuclear weapons.

This dilemma presents difficulties from a policy perspective, but there are measures to mitigate the risks. One conclusion is that message discipline is critical. The US and South Korea could do more in terms of deterrent capability development, deployment and decisionmaking, but message it less publicly—just enough to communicate resolve to North Korea, but not so much as to spook the South Korean population. The core of the Washington Declaration—the establishment of the NCG—is potentially a step in the right direction: it might enhance elite politicians’ sense of reassurance while being boring and wonky enough that it does not reach the radar of average South Korean citizens.

Combined military exercises could be expanded and scaled up to hone readiness but also regularized and normalized such that they are less subject to close media attention. This should go hand-in-hand with the Yoon and Biden administrations generally refraining from escalatory North Korea-directed rhetoric that promises devastating retaliation should North Korea execute its various and sundry menacing attacks. A more convincing overture at talks with North Korea—i.e., going beyond the US-ROK alliance boilerplate that “talks without preconditions” are available—might also balance the dissuasive effect of combined military exercises (which could continue on their present path) with the potentially attractive carrot of meaningful diplomatic exchange. In short, the US-ROK alliance should absolutely carry a big stick, but it should also speak more softly.

  1. [1]

    E.g., even if North Korea were hit with nuclear weapons by the US in retaliation for its own first strike against South Korea, North Korea would have a residual capability to strike the US in return for the retaliation.

  2. [2]

    One notes here that the current state of US and South Korean missile defense—both in/around the Korean Peninsula, as well as for protecting US territory—is considered insufficient to neutralize North Korea’s nuclear arsenal.

  3. [3]

    Additional, non-mutually exclusive possibilities include fears about China (notably in the context of regional great power competition with the US) and a desire for the international prestige of being a nuclear weapon state.

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