The Peninsula

Alternative Futures: ROK Nuclear Weapons and the U.S.-ROK Alliance

This piece is one of 12 contributions to KEI’s special project on South Korea’s nuclear armament debate that will run on The Peninsula blog over the next month. The project’s contributors include young, emerging, and mid-career voices, examining the debate from a historical, a domestic, and an international perspective. On Wednesday, March 15, KEI will host a conference featuring our various contributors’ work at our Washington, D.C. office and launch a compilation of all the pieces in a single, special KEI publication.

An overt, formal decision by Seoul to plan or prepare for the acquisition of an independent nuclear weapons capability by all accounts remains firmly in the realm of the hypothetical as of February 2023, despite concerns accelerated by South Korean President Yoon Suk-yeol’s recent remarks. But, such a decision would cause something of a high-stakes “shock” to the system that is the 70-year-strong alliance between the United States and Republic of Korea (ROK). One important question regarding the “linchpin” for peace, security, and prosperity in the Asia-Pacific is thus whether that system is likely to absorb, implode or adapt to such a shock. While possible alternative futures are many, the logic chain explored below suggests that the alliance would ultimately adapt; if Seoul’s calculus could be revised, it could drive enhanced U.S. and ROK investments in extended deterrent laydowns on the Peninsula.

Attuned to the elusiveness of proliferation forecasting, alliance observers and defense experts are hedging their bets. The immediate international reaction to a nuclear ROK (or a ROK on its way to going nuclear) is expected to be overwhelmingly negative, with Seoul’s actions causing major consternation among and push back from fellow signatories of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). Indeed, this reaction would be consistent with the U.S. response to South Korea’s nuclear weapons activities in the 1970s. Beyond the initial nonproliferation crisis, however, robust predictions for the alliance are impossible. This is largely because the alliance is a complex and time-tested relationship, which encompasses a myriad of joint activities, spanning multiple sectors and domains.

The current analysis is structured to examine U.S.-ROK alliance dynamics in the context of an indigenous South Korean nuclear weapons program. In other words, assumptions regarding the uncertainties of whether, how, or on what timeline the ROK government has been directed to implement a decision to move forward with a national program are not of concern. Instead, in the posited setting, we are to assume South Korea has already proliferated, or is perceived to have proliferated.** Against this backdrop, an interrogation of alternative futures must begin with an examination of the key factors that U.S. policy-makers could potentially weigh as they contemplate whether to seek to arrest or reverse their ally’s choice.

The risk of broader instability inherent in an alliance between the United States and a nuclear ROK would likely be too high for support of the program to be a viable option for Washington. It is true that management of the array of probable proliferation penalties – from UN and bilateral sanctions of various types, to the Nuclear Suppliers Group, just to name a few – for violating the NPT would pose a proximate challenge for the two states, made especially complicated by the U.S. stalwart role in leading that regime. Yet while painful, that challenge would be a discrete one. Less predictable and less manageable than a counterproliferation campaign from the U.S. perspective would be two cascades of events that could stem from South Korea’s choice.

First, a South Korean nuclear arsenal would introduce several new drivers of nuclear escalation on or around the Peninsula and – with the DPRK reportedly close to acquiring ballistic missiles with intercontinental reach, if it has not done so already – for the continental United States. These include a heightened temptation for North Korea to execute a preemptive attack against the South Korean program in its infancy, eliminating Seoul’s new capability before it presents too complicated a target set; games of nuclear brinkmanship between two relatively novice nuclear powers; and an exacerbation of the messaging dynamics and chances of misperception, complicated enough between two nuclear powers, let alone three or four given China’s potential involvement.

Second, U.S. acquiescence of a nuclear ROK would set a dangerous precedent for other states covered by the U.S. extended nuclear deterrent guarantee, most of whom would be tracking reverberations of the South’s actions on U.S.-ROK alliance. Like South Korea, several of those states possess the technological wherewithal (i.e., the materials, expertise, or technical capacity, or combinations thereof) to develop a nuclear weapon, but have not needed to leverage it on account of the U.S. commitment. Weak or non-existent opposition from the United States could thus encourage or activate the nuclear ambitions of others, such as Japan or Taiwan. The unravelling of multiple relationships, and subsequent impact on regional security architectures, would threaten to seriously undermine U.S. interests at a time when Washington is seeking to leverage its allies and partnerships to address a deteriorating threat environment, with the “pacing threat” of China at its core.

A hypothetical future that includes a nuclear ROK is thus equally likely to feature a strong and persistent effort by the United States to persuade Seoul to change its mind. By seeking to alter the (supposed) status quo, the United States would by definition be engaging in a compellence campaign, an exercise of coercive diplomacy known to students and practitioners of international relations as more costly and more difficult than the more frequent practice of deterrence.

The upshot: a return to the previously extant state of affairs – with the tools of nuclear statecraft exclusively owned and wielded by the United States – will be viewed as a non-starter by a nuclear South Korea among both the leadership and its populace, if polling and domestic political trends continue. Rolling back the ROK’s nuclear aspirations would only get tougher as the program advances. The job will therefore demand further and more tangible U.S. acknowledgement, or deeper internalization of the motivations behind Seoul’s need to arm itself with nuclear weapons in the first place. The future posed here could be interpreted as a “natural outgrowth” of concerns evident today, rooted in the acuteness of the North Korean missile and nuclear threat, and amplified by an erosion of confidence in U.S., ROK, and alliance tools to curb that threat. If the United States is to maintain support from the ROK in the region to counter China, it would be necessary to find new or revive legacy means to “institutionalize” extended deterrence so that its partner is more vested.

What types of interventions could the United States consider to persuade South Korea to act differently? The counterproliferation toolkit developed and enlisted by the Ford Administration offers a precedent to draw from. While tailored to a very different strategic and normative setting – the threat from North Korea did not include nuclear weapons and multiple classes of ballistic missiles, and the NPT had only recently entered into force – concessions or “carrots” were part of that package. For example, the eschewal of advocacy for troop withdrawals from the Korean Peninsula was purportedly shaped by the South’s signaling that “future behavior would be contingent on American security commitments.”

Based on the ebb and flow of U.S.-ROK extended deterrence practice and alliance management over the past fifteen years, shifting Seoul’s calculus in this postulated future would require an intervention rooted in the U.S. nuclear triad. This is somewhat disappointing, considering how widely the relationship has broadened over the past twenty years; what began as an exclusively nuclear umbrella now includes extensive engagement in other domains, such as conventional strike and missile defense. Though the sources of strength for U.S.-ROK relations have multiplied beyond nuclear issues, the chances are that non-nuclear concessions (e.g., expanded cooperation in space, for example) will not redress the perceived gap in the U.S. security guarantee. While the Department of Defense’s concept of integrated deterrence may ultimately revamp this trade space, a nuclear solution would be needed to address a nuclear problem.

The most direct way to reverse South Korea’s pursuit of an independent nuclear program would likely center on “hard power” indicators of nuclear resolve, that is, the physical hardware the United States commits to nuclear deterrence on South Korean territory. Nuclear assets in theater dwindled in the early 1990s, when U.S. nuclear weapons were fully withdrawn from the Peninsula as part of the Presidential Nuclear Initiatives. Presidents Biden and Yoon have publicly reopened the debate over forward nuclear deployment, through the joint reaffirmation of “the commitment of the U.S. to deploy strategic U.S. military assets in a timely and coordinated manner as necessary, as well as to enhance such measures and identify new or additional steps to reinforce deterrence in the face of DPRK destabilizing activities.” The administrations reiterated this commitment at the defense ministerial meeting in Seoul last month.

In theory, the United States could thus implement POTUS’ direction and offset its ally’s perceived need by beginning to lay the groundwork for the South Korean military to assume some role in the air-breathing leg of the U.S. nuclear triad, currently consisting of a combination of heavy and multi-role stealth bombers, gravity bombs and cruise missiles. The negotiation of a burden-sharing agreement could cover the reintroduction of gravity bombs to the Peninsula (which could be handled unilaterally or bilaterally), host nation support, and forward deployment of U.S. capability on an allied or dual-operated airfield.

The practicalities of such a process would be non-trivial, demanding no less than the resurrection of an infrastructure for the safe, secure, and NPT-compliant storage of U.S. nuclear weapons on the Peninsula. A complementary set of muscle movements could also be necessary to ensure the U.S. and ROK air forces could be sufficiently trained, organized, and equipped for the deterrent mission (not to mention certified). The precise form of the burden-sharing arrangement would be further circumscribed by capacity realities on both sides. All such efforts would need to be nested within the existing U.S.-ROK arrangement on the Peninsula without sacrificing the readiness of extant forces. All such efforts would also demand careful attention to the concerns of the nonproliferation community (as well as the reaction by the North Korean regime). The alliance would thus incur many types of costs for enhancing its extended deterrent posture, but altogether a worthy bargain in light of the cascading risks a nuclear ROK could set in motion.

Caroline R. Milne is a Research Staff Member in the Strategy, Forces, and Resources Division at the Institute for Defense Analyses (IDA). The views, opinions, and findings expressed should not be construed as representing the official position of either the Department of Defense or IDA. For helpful comments and suggestions, the author thanks Bill Chambers, Tom Greenwood, Kyung-joo Jeon, Alex Shykov, and David Stein.

Photo from U.S. Secretary of Defense’s photostream on flickr Creative Commons.

** It is important to acknowledge that the development of an independent nuclear weapons capability can take many forms and typically comprises many steps. Previous instances of South Korean nuclear proliferation-related behavior included, for example, feasibility studies of nuclear weapons development and interest in importing reprocessing technology; today it might look different. It is the view of this author that while the form of South Korea’s proliferation could impact the speed or degree of the U.S. response, it would not impact the core U.S. objective of rolling back the program.