In January, the U.S.–South Korean alliance was rocked by President Yoon Suk-yeol’s surprising suggestion: his country, a law-abiding member of the international system and a key U.S. ally, might need its own nuclear deterrent. In the United States, many foreign policy experts were aghast. A South Korean nuclear arsenal was unnecessary, they argued, because Seoul enjoys U.S. protection. Moreover, a South Korean nuclear program would violate the Nuclear Nonproliferation Treaty (NPT), destabilize East Asia, and expose South Korea to crippling economic sanctions. A few days later, Yoon backpedaled, declaring that the country did not need nuclear weapons after all. The furor appeared to have subsided.

The problem is that Yoon’s observation was fundamentally correct. The nuclear balance on the Korean Peninsula is changing in a profound way. Thanks to North Korea’s development of long-range nuclear capabilities, U.S. cities will soon be in the cross hairs during any war on the peninsula. In the past, Washington could use the threat of a U.S. nuclear response to deter North Korea from using nuclear weapons against the South, because in a worst-case scenario, it would be Seoul and Busan at risk rather than New York and Los Angeles. But North Korean missiles will soon be able to reach the entire United States—forcing U.S. policymakers to consider how much they wish to wager on behalf of their ally. As a result, the U.S. nuclear commitment to South Korea is increasingly founded on promises that are unlikely to be kept in the case of a crisis. Mere reassurance is no longer enough for Seoul, and the United States and South Korea need to update their strategy for nuclear deterrence on the peninsula.

The solution to this problem lies in the history of Europe during the Cold War. When the Soviet Union deployed nuclear weapons that could reliably strike the United States, NATO allies became nervous. They began to question whether Washington would risk its own citizens to defend Europe. Those fears were not quelled by renewed pledges of solidarity from Washington. Instead, the United States’ NATO allies chose from a set of options along a “nuclear continuum,” from forward-deployed U.S. nuclear weapons, to nuclear sharing, to independent European nuclear arsenals. These policies have helped keep the peace in the decades since. Today, those same options are available to South Korea. Seoul should work with Washington to strengthen its deterrence of Pyongyang and provide a stronger foundation for the U.S.–South Korean alliance over the long term.

ALLIANCE IN THE CROSS HAIRS

South Korea is an important ally of the United States. Located in a strategically vital region of the world, it is a vibrant democracy with a thriving market economy and a leading technology industry. It welcomes U.S. forces into its country to live and train in peacetime, and it is notable among U.S. allies in its substantial contributions to its own defense.

North Korea’s acquisition of nuclear weapons, in 2006, created a crisis for Seoul and Washington—but did not challenge the basic logic of the alliance. Nuclear-armed or not, if North Korean forces were to invade South Korea, the vastly superior U.S. and South Korean conventional militaries would be able to stop them. And if North Korea were foolish enough to use nuclear weapons, the United States would retaliate. North Korea could bluster, but launching a large-scale military attack would be self-defeating. Pyongyang was boxed in.

North Korea’s recent nuclear developments, however, pose a far more serious threat to the alliance. Having started with a handful of small fission bombs, North Korea is now developing vastly more destructive thermonuclear weapons. Pyongyang is also testing intercontinental ballistic missiles that could reach the United States. A decade ago, defending South Korea required the United States to risk the lives of U.S. military personnel—a high price, but one that Washington was willing to pay. Today, defending South Korea risks the loss of several U.S. cities.

War could erupt on the Korean Peninsula as a result of calculated aggression by Pyongyang, an accident, misperceptions, or even a partial collapse of the North Korean regime that forces South Korea to move north. Regardless of the conflict’s origins, the North Korean government would feel powerful pressure to reach for its nuclear weapons. These could be used offensively, say, to destroy South Korean conventional forces. More likely, they might be used defensively—to force South Korea and the United States to halt the war before they could march on Pyongyang. South Korea depends on the U.S. nuclear umbrella to deter North Korean nuclear escalation. But as cities across the United States fall within range of Pyongyang’s missiles, South Koreans are, quite reasonably, questioning Washington’s assurances that it will stand beside Seoul no matter the costs. North Koreans have undoubtedly made the same calculations, which is why Pyongyang invested in developing missiles that can reach the U.S. homeland.

These changes are unfolding gradually. Pyongyang’s intercontinental capabilities are still immature. Moreover, as long as the United States has effective counterforce capabilities, Washington can retaliate against a North Korean nuclear attack on South Korea with a strike aimed at Pyongyang’s remaining nuclear arsenal. But as the range, reliability, and number of North Korea’s missiles increase, the contradictions at the heart of U.S. nuclear commitments to Seoul will become harder to reconcile. South Korea and the United States need to find a new strategy for deterrence on the peninsula.

NATO’S SOLUTION

After World War II, the United States and its European allies faced a dilemma that mirrors the problem that Washington and Seoul confront today. From its founding in 1949—when Soviet conventional forces were superior—NATO relied primarily on U.S. nuclear weapons to deter a Soviet attack. U.S. nuclear threats were initially credible because the Soviet nuclear arsenal could not reach the United States. If the Soviets attacked Europe with nuclear or conventional forces, the Soviet Union would be destroyed—and the United States would emerge nearly unscathed.

But the credibility of U.S. nuclear guarantees eroded. The Soviet Union developed bombers that could reach the United States and later built intercontinental missiles. It crossed a key threshold in the early 1960s, when it developed an arsenal that U.S. forces could no longer confidently disarm. The once credible U.S. nuclear threat was now in doubt. Why, after all, would the United States retaliate against a Soviet attack on Europe if doing so would trigger a nuclear counterattack against its own cities? As French President Charles de Gaulle asked U.S. President John F. Kennedy, would the United States really “trade New York for Paris?” Kennedy and his successors sought to assure their allies that they would, but European leaders were unconvinced. The calculus of deterrence in Europe had changed.

So the alliance adapted. NATO developed strategies to convince the Soviets that any major attack would lead to nuclear war. First, the United States maintained large numbers of tactical nuclear weapons in Europe for use in wartime. These weapons enhanced the credibility of the alliance by giving the president options to defend NATO without initiating a full-scale strategic nuclear war. An invasion by the Soviets would trigger tactical nuclear strikes on their military forces, a more credible response than a suicidal U.S. nuclear strike on the Russian homeland. U.S. tactical nuclear weapons remain in Europe today.

A second strategy to strengthen nuclear deterrence in Europe was sharing nuclear weapons with NATO allies. Under this plan, the United States stored nuclear weapons in Europe for Europeans to use in the event of a major war. European military forces were equipped and trained to deliver U.S. nuclear weapons. And most important, the United States promised to transfer control of the weapons to its allies if war was imminent. In principle, this allows European governments to use nuclear weapons to defend their countries even if U.S. resolve wavers in the face of Soviet threats. And because the weapons remain under U.S. control in peacetime, the policy does not violate the NPT, which bars nonnuclear states from controlling nuclear weapons.

Two NATO members pursued a third strategy: acquiring their own nuclear arsenals. The United Kingdom developed nuclear weapons in 1952, several years before long-range Soviet bombers put the United States at risk. London’s decision was partly animated by concerns about the durability of the U.S. commitment. France’s decision to deploy its own arsenal in 1960 was also fueled by doubts about the U.S. pledge to defend Europe.

NATO’s deterrence problem during the Cold War mirrors South Korea’s conundrum today. South Korea depends on the United States to deter a nuclear attack—but it would be monumentally risky, or even irrational, for the United States to carry out its nuclear commitments to Seoul if doing so could result in the destruction of American cities. Confronted with this problem half a century ago, European allies decided that they needed more than verbal assurances and symbols of commitment from the United States so they adopted policies across the nuclear continuum. South Korea should consider how NATO’s deterrence playbook might apply today.

CHOICES, CHOICES, CHOICES

South Korea and the United States have a variety of options. At one end of the continuum, the United States could simply do more to signal its commitment to the alliance, for instance, by sending more aircraft carriers and submarines to visit South Korean ports or flying more nuclear-capable aircraft over the peninsula. U.S. and South Korean officials are now reportedly talking in greater detail about nuclear plans—something that Washington had resisted in the past. Some U.S. analysts have proposed creating a U.S.–South Korean version of NATO’s Nuclear Planning Group to formalize discussions about deterrence on the Korean Peninsula.

These steps, however, are largely symbolic and fail to address the actual deterrence problem. The challenge to deterrence is not a lack of closeness in the U.S.–South Korean relationship but the fact that retaliating on behalf of its ally would require the United States to risk its own destruction. Such promises would be viewed with incredulity, even when issued to close friends. In the 1950s, the United States had no closer allies than its European partners, but Washington’s assurances were not enough to convince them of its commitment to their security.

An option further along the nuclear continuum would be for Seoul to request the deployment of U.S. nuclear forces to South Korea, which hosted a small arsenal of U.S. tactical nuclear weapons during the Cold War. Such forward basing would signal Washington’s readiness to use those weapons if needed, and it would give a U.S. president more choices for tactical strikes were North Korea to employ nuclear weapons against the South. This option, however, would ultimately run up against the same limitation: engaging in a tactical nuclear war in South Korea’s defense would still put the U.S. mainland in grave danger.

Another option along the nuclear continuum is nuclear sharing, modeled on the NATO policy. This approach would involve stationing U.S. weapons in South Korea, equipping South Korean aircraft to carry them, and training South Korean pilots to use them. Most important, a nuclear sharing agreement would entail a promise from the United States to transfer control of those weapons to South Korea in the event of a major attack. In one sense, nuclear sharing would make deterrence more credible. If, during a crisis, Washington transferred weapons to Seoul’s control, North Korea should expect the South to defend itself if attacked. But South Koreans might doubt whether the Americans would actually give them the weapons. Sharing nuclear weapons with South Korea therefore rests on the tenuous assumption that U.S. leaders are too cautious to use nuclear weapons to retaliate against a North Korean attack but sufficiently bold to pass those weapons to South Korea in the midst of a crisis.

Nuclear sharing is not a perfect answer to the growing deterrence problem, but it might be part of a solution. It may, for example, be the option most likely to generate diplomatic support from key partners. NATO members who benefit from their own nuclear sharing agreements may hesitate to criticize the same plan for South Korea—an NPT member in good standing facing threats from a nuclear-armed pariah. Furthermore, a nuclear sharing arrangement could be seen as a provisional step toward an independent nuclear capability. The arrangement would provide South Korea with valuable training and planning for nuclear operations. In this sense, nuclear sharing is both an alternative to an independent South Korean deterrent and a warning that North Korea’s actions are pushing Seoul toward possessing the bomb.

The final option on the nuclear continuum is the one taken by the British and French during the Cold War: the development of an independent nuclear arsenal. This option would maximize deterrence on the Korean Peninsula because Pyongyang would understand that Seoul would retaliate in the event of any North Korean use of nuclear weapons against the South. A 2023 poll found that over 70 percent of South Koreans favor possessing an independent nuclear arsenal.

Importantly, this option is legal under the NPT. The treaty declares that any member can withdraw if “extraordinary events, related to the subject matter of this Treaty, have jeopardized the supreme interests of its country.” The threat from North Korea easily clears the bar for Seoul’s legal withdrawal from the treaty. North Korea violated the 1992 Korean denuclearization agreement, several legally binding UN Security Council resolutions, and the 1994 Agreed Framework, in which it had committed to freezing its nuclear weapons development program. North Korea continues to test nuclear weapons and missiles, in defiance of further Security Council resolutions. All the while, North Korean leader Kim Jong Un regularly issues nuclear threats against South Korea and its partners. South Korea would simply need to give three months’ notice, and then it would be legally free to emulate France and the United Kingdom and build a modest arsenal in order to deter nuclear attacks.

The two middle options—forward-deployed U.S. nuclear weapons and the sharing of nuclear weapons—are the best of the choices available to Seoul. South Korea needs more than symbolic assurances from the United States. And an independent nuclear capability might be too abrupt a change all at once, given the global norm against proliferation. This choice, however, is one for South Korea to make, as a sovereign, law-abiding state—ideally, in consultation with Washington.

WEIGHING THE OPTIONS

U.S. national security experts increasingly acknowledge the growing deterrence challenges on the Korean Peninsula but oppose all but symbolic changes. Critics of nuclear sharing and an independent South Korean nuclear arsenal argue that these options would constitute expensive misdirections of resources away from conventional armed forces.

But South Korea has already paid much of the cost for a nuclear arsenal. It has 14 military airfields and hundreds of hardened aircraft shelters, which are suitable for storing nuclear weapons. It also owns dozens of F-15 and F-16 fighter aircraft, which can be used to carry nuclear bombs. And notably, it is deploying a fleet of ballistic missile submarines, currently armed with conventional weapons. These submarines’ missiles can be modified to carry nuclear warheads. For any nuclear country, a large portion of the expense of nuclear capabilities lies in the development of delivery systems. South Korea has already purchased many of these weapons.

Other critics, such as Bruce Klingner, a Korea expert at the Heritage Foundation, warn that withdrawal from the NPT would have major consequences for the economy of South Korea. The country would lose access to commercial nuclear exports from members of the international Nuclear Suppliers Group, with serious implications for South Korea’s civilian nuclear energy program. Nuclear power accounts for 15 percent of South Korea’s energy consumption, meaning that disruptions would require increased reliance on fossil fuels and renewables. 

Withdrawal from the NPT, however, would be necessary only if South Korea chose to develop an independent nuclear arsenal instead of another option along the nuclear continuum. Furthermore, if Seoul did withdraw from the NPT, the economic disruptions could be weathered. Indeed, punishment by the Nuclear Suppliers Group could even strengthen South Korea’s civilian nuclear industry in the long term. India faced years of exclusion from NSG supplier networks, which according to the nuclear scientist Siegfried Hecker spurred innovation and helped India become a leader in certain nuclear technologies. And in 2008, New Delhi received a waiver from the NSG and is now fully integrated into commercial nuclear markets. South Korea—were it to develop its own arsenal—would, as a law-abiding member of the international community, be a strong candidate for a waiver.

One concern commonly invoked in debates about South Korean nuclear weapons is the risk that Seoul’s new capabilities could lead to a regional cascade of proliferation. In particular, South Korea’s nuclearization might cause Japan to reconsider its own nonnuclear stance. But Japan has consistently rejected nuclear weapons despite years of threats from Pyongyang. It is difficult to argue that Tokyo, which has remained resolute in the face of Pyongyang’s menacing words, would panic in response to a democratic South Korea’s restrained, legal steps to acquire nuclear weapons.

Many South Korean leaders worry that acquiring nuclear weapons would make their country a pariah like North Korea. But no fair-minded observer could equate the policies of Seoul and Pyongyang. North Korea, one of the world’s most brutal tyrannies, developed its nuclear arsenal illegally, first violating its NPT agreements and then ignoring UN Security Council resolutions that prohibited its nuclear arsenal. South Korea, on the other hand, is a democratic, law-abiding country that clearly prefers not to possess nuclear weapons. Moving to acquire an independent force legally, and in response to the threats of its neighbor, is entirely justified. Leaders in France, Germany, the United Kingdom, and elsewhere would have a hard time explaining why they should be allowed to have nuclear weapons (independently or through nuclear sharing) while South Korea must live in the shadow of a totalitarian state’s nuclear threats.

China would be angry and would likely punish South Korea because Beijing opposes the spread of nuclear weapons, especially to a U.S. ally with whom it shares a border. Beijing has frequently hit countries such as Australia, Norway, the Philippines, and others with economic sanctions to coerce or punish them for various policies it opposes. Notably, China sanctioned a South Korean corporation, Lotte, when it cooperated with Seoul in deploying a U.S.-built missile defense system in 2017. Indeed, the costs of damaged relations with China should be a core consideration for South Korean leaders as they decide how far along the nuclear continuum they must move. But such costs—which South Korea’s friends may be able to help mitigate—are preferable to continued vulnerability to North Korean aggression. Taking moderate steps to strengthen deterrence would give South Korea protection and flexibility if relations with the North were to worsen.

SEOUL STEPS UP

South Korean diplomats and analysts often ask why the United States treats South Korea differently than it does its European allies. But this question rests on a faulty premise: throughout most of the nuclear age, the United States opposed proliferation, even among its closest European allies. NATO nuclear sharing was conceived as an alternative to European NATO members acquiring their own nuclear weapons. In Asia today—as in Europe during the Cold War—the United States will oppose any options along the nuclear continuum right up until compromise is required to keep U.S. allies from simply going nuclear on their own.

The good news is that the growing deterrence challenges, if managed well, could bring Washington and Seoul closer together. For years the United States has sought South Korean participation in broader regional security efforts. Yet South Korea remains outside the Quadrilateral Security Dialogue made up of Australia, India, Japan, and the United States. Seoul is reluctant to pursue closer cooperation with Japan and appears unenthusiastic about joining the United States, Japan, and the Netherlands in denying critical dual-use technologies to China. Seoul has preferred to keep the alliance focused on North Korea and the threat posed by its nuclear advances. If South Korea now needs more help from the United States in the realm of nuclear deterrence, Seoul should reciprocate with greater support for broader U.S. interests in the region.

Whatever deal they might strike, Seoul and Washington should confront the growing deterrence problem together. They should explain that the source of the problem is North Korea’s illegal nuclear arsenal and the Kim regime’s threats to use it. If Pyongyang decides to reduce its own nuclear capabilities, South Korea’s efforts to nuclearize could be shelved. But if not, reasonable South Korean leaders may decide to take meaningful steps toward further nuclearization. And the world should understand.