At their joint press conference on Wednesday, South Korean President Yoon Suk-yeol and U.S. President Joe Biden did not miss the opportunity to remind us that their countries’ alliance of 70 years is “ironclad.” It is an apt metaphor, both because iron is strong and because iron rusts. New measures announced in the two countries’ Washington Declaration could galvanize the alliance against declining confidence in extended deterrence and rising calls for nuclear proliferation—if the allies use them to pivot away from a corrosive reliance on nuclear weapons.
At their joint press conference on Wednesday, South Korean President Yoon Suk-yeol and U.S. President Joe Biden did not miss the opportunity to remind us that their countries’ alliance of 70 years is “ironclad.” It is an apt metaphor, both because iron is strong and because iron rusts. New measures announced in the two countries’ Washington Declaration could galvanize the alliance against declining confidence in extended deterrence and rising calls for nuclear proliferation—if the allies use them to pivot away from a corrosive reliance on nuclear weapons.
At a time when the alliance’s deterrence posture toward North Korea has never been stronger, there is a real crisis of confidence in both countries. The harrowing years of the Trump administration, when an erratic president extorted payoffs from Seoul and regularly threatened to break the alliance, and North Korea’s development of intercontinental ballistic missiles have caused South Koreans to worry that Washington could abandon them in a conflict to try to protect U.S. cities. North Korea’s new arsenal of tactical nuclear weapons raises difficult questions about how the alliance would respond to limited nuclear use, another way that Pyongyang is trying to split the alliance.
One manifestation of the rust threatening the alliance has been the surge in calls for Seoul to acquire nuclear weapons. In January, Yoon broke a taboo on official discussion of nuclear options by musing publicly, “If problems become more serious, South Korea could deploy tactical nuclear weapons here, or we could acquire our own nuke as well,” noting that “it would not take long.” The comments alarmed U.S. officials, who know that a South Korean nuclear weapons program would leave their ally exposed to preventive attacks, economic coercion, and diplomatic isolation—to say nothing of the damage it would do to the enduring U.S. interest in preventing the proliferation of nuclear weapons.
As part of his state visit to Washington, Yoon gave a strong statement in support of the bargain at the heart of the alliance and reaffirmed South Korea’s commitment to the Nuclear Nonproliferation Treaty. He called the summit an “unprecedented expansion and strengthening of the extended deterrence strategy” and predicted that “any concerns that Koreans may have against North Korean nuclear weapons will be relieved.”
We can hope that Yoon can follow his statement with action to shape public opinion in Seoul to quiet calls for nuclear armament. But the politics of nuclear armament in Seoul are complex and unlikely to change without a concerted effort. The pro-nuclear camp in South Korea is dominated by zealots who are personally committed to nuclear weapons as a matter of national strength and based on a simplistic adage that “only a nuke can deter a nuke.” They are joined by populists primarily motivated by public opinion polling that shows majority support for nuclear weapons. Finally, there are alliance bargainers who are seeking to use the threat of proliferation to extract concessions from the United States, mainly in the form of nuclear assurances. In the United States, these arguments have caused general concern, save for a few academics and conservative contrarians who won’t quite say they are advocating for nuclear proliferation.
None of these groups have thought in detail about when and why nuclear use might be required to defend South Korea, and all of them tend to minimize the enormous costs and risks that a nuclear program would entail. Nevertheless, these scattered calls for nuclear proliferation have posed a serious risk to the alliance. They force U.S. officials to turn their attention away from deterrence to nonproliferation while distracting from the more important problems posed by North Korea and China. Worse, they suggest a weakness in the alliance and in the confidence in conventional military capabilities to deter North Korean coercion. And they signal that South Korea doubts alliance cohesion, inviting North Korean efforts to drive a wedge between the allies.
Pro-nuclear discourse in Seoul is not the only spot in the alliance showing signs of rust. As North Korea expanded its nuclear forces, South Korea invested heavily in conventional capabilities to rapidly strike the regime’s leadership, command and control facilities, and nuclear forces, including F-35 aircraft and an arsenal of sophisticated missiles. U.S. officials have little visibility into when or how Seoul would order these strikes, which could rapidly escalate a conflict if North Korean leader Kim Jong Un believes he must use his nuclear forces before he loses them in a South Korean attack.
For more than a decade, both governments have believed that the solution to the alliance’s problems with cohesion and deterrence could be found in the U.S. nuclear arsenal. Preventing nuclear proliferation is one of the three central roles for U.S. nuclear forces. The theory is that shows of strength with nuclear bombers and meetings between nuclear policy officials can convince South Koreans that the United States is ready and willing to use nuclear weapons to protect them.
However, nuclear assurance never actually seems to assure South Koreans but only leads to more and more demands for nuclear assurance. U.S. officials say it feels like pouring water into a bucket with a hole in the bottom. Already, some South Korean experts are saying that, even though the Washington Declaration delivers what they had asked for, it won’t be enough to satisfy the bargainers or the zealots.
Indeed, there is growing evidence that nuclear assurance is doing more to fuel the alliance’s problems than to fix them. The reason is that U.S. officials can never prove that they have the resolve to use a nuclear weapon. Nuclear assurance can demonstrate capability but can never prove resolve. The reason is that nuclear weapons would be used only as a last resort, in the event that the United States had no effective conventional option available. The United States would not promise to use a nuclear weapon in advance because it might not be in either country’s interest. This is why the Washington Declaration promises a “swift, overwhelming and decisive response”—but not necessarily a nuclear response.
Instead, nuclear assurance contributes to the misperception that the alliance is dependent on nuclear weapons, feeding the unfounded belief of the zealots that only a nuke can deter a nuke. This only encourages South Koreans to believe they need their own nuclear weapons. It gives bargainers an opportunity to press for more U.S. measures, which in turn gives more ammunition to the zealots. For this reason, steps that increase the salience of nuclear weapons in the alliance run out of the bottom of the bucket and corrode the alliance further.
To break this cycle of anxiety, the allies will have to reduce their reliance on nuclear use to deter North Korea. The credibility and cohesion of the alliance depend first and foremost on conventional forces that can work in concert to deter and respond to any North Korean attack. In this regard, the initiatives announced during Yoon’s state visit could be key to repairing the rust in the alliance.
First, Biden promised to consult with Yoon after a North Korean nuclear attack, an important innovation that can improve crisis coordination in ways that benefit both countries. Seoul will gain confidence that it will not be sidelined, and Washington will have a better window into South Korean military operations and their implications for conflict escalation. Seoul’s preferences and plans should bear heavily on any presidential decision on nuclear use. The idea should be formalized in the nuclear authorization process and extended to other allies.
Second, the joint statement also highlighted and expanded a new series of tabletop exercises that examine responses to a North Korean nuclear attack. Closely examining potential nuclear contingencies will reveal not only that U.S. officials are resolved to stand with their ally in any conflict but also that nuclear use would have enormous costs and risks that both countries will prefer to avoid if possible. They will also underscore the importance of robust conventional military response options and tighter alliance coordination in escalation planning.
Third, a new nuclear consultative group can provide another opportunity for improved coordination on deterrence strategy. However, there is a risk that if the group focuses only on nuclear weapons, it will perpetuate the harmful addiction to nuclear weapons and associated assurance problem. Any alliance mechanism to deter coercion and limit escalation that fails to place conventional forces at the center of its strategy is doing more harm than good. The group should concentrate on strengthening the alliance’s ability to deter and respond to a nuclear attack by integrating U.S. and South Korean conventional forces into a cohesive strategy. The joint statement’s commitment to connect South Korea’s new Strategic Command to existing planning processes is an important step in the right direction.
If implemented correctly, the announced steps can reduce the alliance’s reliance on nuclear weapons and shift focus to the conventional capabilities that are the core of its deterrence posture. Prioritizing and valuing the alliance’s conventional forces can create a stronger deterrent and a stronger alliance. Over the long run, it is the only way to reduce nuclear proliferation risks in Seoul.
But there is cause for concern. Already, media have focused on the nuclear aspects of the announcement, including the U.S. offer for a ballistic missile submarine to visit a South Korean port. It is a “nuclear,” not a “strategic deterrence,” consultative group. And Yoon said incorrectly that the United States had promised that a North Korean nuclear attack would be met with “the full force of the alliance, including the United States’ nuclear weapons,” the latest in a string of comments that have overstated U.S. commitments.
If the steps today encourage South Korea to fixate on nuclear weapons, they will end up like past nuclear assurance measures—just more water poured into the bucket and out the hole in the bottom. That water will continue to rust the ironclad alliance and lead to a peninsula that is less stable and less secure.
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