Nuclear Nuclear Assurance of South Korea: One step forward, two steps back

Adam Mount is a Non-Resident Senior Fellow at the Federation of American Scientists.

For the last two decades, the US commitment to extend nuclear deterrence over South Korea has faced increasing pressure. In that time, seven North Korean nuclear tests and rapid advancements in a diverse array of advanced missile technologies have forced the alliance to adapt its posture and develop new capabilities and concepts to deter aggression and nuclear first use. For the most part, the alliance has met this challenge. Improvements in conventional capabilities—especially South Korean stand-off strike systems—has tipped the balance of power further south over the same period, providing additional capability not only to deny North Korea the benefits of aggression but also to deter and respond to nuclear use.

While the alliance is meeting the military challenge, it is failing to manage the political challenge of extended deterrence. Over the last two decades, South Korean confidence in US security guarantees has plummeted, despite this strong military position. Two trends have backed the alliance into a corner. First, the US commitment to defend South Korea with nuclear weapons if necessary has become cancerous, threatening to metastasize and infect other aspects of the alliance. Second, the Trump administration’s statements and actions forced Seoul to confront the risks of depending on an ally that is grappling with isolationist authoritarian tendencies.

US defense officials have considered nuclear extended deterrence as part of the framework of the nation’s alliance structure. The theory is that extended deterrence assures allies of their own security, meaning that they need not consider developing their own nuclear arsenal. The problem is that, because the President of the United States retains sole authority over the use of nuclear weapons, no ally can be certain that Washington will use a nuclear weapon in and only in cases when they believe it is necessary. When allies express anxiety about depending on US extended nuclear deterrence, US officials customarily embark on a round of nuclear assurance, displaying nuclear-capable systems and holding consultations designed to show allies that they are attentive and committed to the nuclear mission.

Especially in South Korea, nuclear assurance not only fails to address allied anxieties, but, in practice, also fuels them. Existing nuclear assurance mechanisms can establish US capability, but cannot prove the resolve of US leaders or provide South Korean officials with a reliable expectation about when, where, and why the United States might use a nuclear weapon on the peninsula. Because there is nothing that US officials can do to resolve either problem, extended nuclear deterrence is an unreliable foundation for an alliance. Worse, nuclear assurance mechanisms effectively raise the salience of nuclear weapons, contributing to a public and elite misperception that South Korea’s security depends on US nuclear weapons, even though this is less and less true by the year as the alliance’s conventional capabilities continue to improve.

South Korean officials and experts have presented a number of proposals that they believe would alleviate their anxieties. The current idea is a request that the United States commit to use a nuclear weapon in response to North Korean nuclear first use, what we might call automaticity. While US officials value ambiguity in nuclear weapons policy to allow the president a range of options (and to avoid internecine policy debates), the prevailing theory in Seoul is that this kind of ambiguity weakens deterrence and signals wavering commitment from Washington. To be clear, Washington would never make this commitment. The US president reserves the right to decide on nuclear employment in a specific contingency because they would ideally consider an array of important factors specific to the crisis; because they would almost certainly prefer to find an effective alternative to nuclear use; and because they would want to consider South Korea’s preferences which may disapprove of US nuclear use on the peninsula, especially under another president. When he was inevitably rebuffed, South Korean President Yoon Suk Yeol apparently stated his preferred policy on behalf of the alliance at the White House, effectively bluffing with US nuclear weapons. Unfortunately, this was the latest in a pattern of misstatements of alliance nuclear policy by President Yoon or senior members of his administration. Together, they signal increasingly severe friction in the alliance.

Other schemes range from forward deployment of US nonstrategic nuclear weapons to the peninsula, offering South Korean officials a role in US nuclear planning, or the development of a nuclear-sharing arrangement that may or may not resemble the one in NATO. The critical feature of all such proposals is that none of them would provide South Korean officials with additional information about when, where, and why the United States would use a nuclear weapon on the peninsula and so would not dispel their anxieties.

Toby Dalton, co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, has presented a helpful taxonomy of the differing logics behind calls for additional nuclear integration within the alliance. Zealots believe that only a nuke can deter a nuke and, for the most part, will never feel secure until a South Korean president of their own party has release authority over nuclear weapons. Bargainers may hold real anxieties over extended deterrence, but exploit them in order to extract concessions from the United States. Populists simply support nuclear proliferation or nuclear sharing as a kind of shibboleth to appear hawkish on foreign policy. Zealots have dominated the conversation, coopting the other groups to push for an indigenous nuclear weapons program or nuclear sharing. Few officials, in Seoul or in Washington, have confronted this narrative.

This fixation on nuclear weapons—to the detriment of other capabilities, including South Korea’s own exceptional conventional defense and deterrence posture—is corrosive to the alliance. Further increasing the salience of nuclear weapons cannot address anxieties, only fuel them. If US officials cannot shift the alliance’s attention to nonnuclear deterrence and educate South Korean public opinion to support the shift, demand for nuclear sharing and nuclear proliferation will continue to rise.

It was in this context that Yoon and the administration of US President Joe Biden a launched the Washington Declaration in April 2023. The summit was essentially the swan song of nuclear assurance. It established a new Nuclear Consultative Group (NCG), expanded tabletop exercises between the allies on nuclear weapons issues (including with U.S. Strategic Command), and issued a presidential commitment to make every effort to consult with South Korea’s president before using a nuclear weapon on the peninsula. Also this spring, South Korean officials visited the US SSBN base at King’s Bay and the SSBN USS Kentucky made a port call in Busan.

Little is known publicly about the composition or agenda of the NCG or why it provides additional value relative to the constellation of existing deterrence consultative groups (including the DSC, EDSCG, and SCM). The South Korean hope is that it will serve as a kind of combined nuclear planning cell in peacetime and mechanism to consult on nuclear decisions in wartime, both of which are unlikely. Though the United States should conduct hypothetical nuclear planning with South Korea (to include target selection and weaponeering, with approximate parameters), US officials still carry an allergy to this kind of activity. Though detailed planning might suffice to educate South Korean officials that nuclear weapons have significant limits in their ability to hold mobile, hardened, and urban targets at risk and cannot magically resolve any crisis, US officials would hesitate to present this information on the grounds that it may be construed as a lack of resolve to use nuclear weapons. Proposals to develop a standing trilateral consultative mechanism with Japan have, unfortunately, languished.

The commitment to consult prior to nuclear use is the least prominent, but the most promising, development. The step is qualitatively different to existing nuclear assurance mechanisms because it directly addresses South Korea’s concerns: that it will not have input into a US president’s decision on nuclear use. Though Seoul will never hold the launch keys to US nuclear weapons, the opportunity to have input into the decision conference is the first known adjustment to the nuclear authorization process in decades and distinctive among US allies. US officials should continue to emphasize and expand on the idea: to formalize it as a step in the nuclear authorization procedure; to establish a secure video, voice, and data channel to enable detailed consultations in a crisis; and to rehearse the link and publicize the results. Ideally, the activity would encourage the South Korean NSC and MND to begin their own planning process to compare nuclear and nonnuclear response options, which can also better prepare President Yoon for this kind of consultation.

These are the twin failures of the Washington Declaration—the first, to assure South Koreans; the second, to reduce the salience of nuclear weapons in the alliance. Nuclear assurance was bound to fail. The summit is the swan song of nuclear assurance because there is little more it could hope to accomplish without transitioning into nuclear sharing, which is unlikely.

Breaking the alliance’s addiction to nuclear weapons is politically and bureaucratically difficult but relatively straightforward. US officials would renovate existing consultative deterrence mechanisms to ensure that they contain not only nuclear but also a range of types of US officials from across the interagency; would emphasize US and South Korean conventional capabilities, including in high-profile trips to the region; to be clear with South Korea’s public about the extremely limited role nuclear weapons play in their defense; and, at the same time, to confront the nuclear zealots in Seoul.