Earlier this month, South Korean President Yoon Suk-yeol set off alarms. In an off-the-cuff remark, he warned that Seoul might need to develop nuclear weapons—or demand redeployment of U.S. nuclear arms to the Korean Peninsula—to counter North Korean nuclear threats. In doing so, Yoon spotlighted a popular view once reserved for hawkish commentators, defense intellectuals, and former military officials. Keeping nuclear weapons out of South Korea will ultimately be a U.S. responsibility that requires addressing both the deteriorating security environment and the domestic drivers underlying Yoon’s statement.

Nuclear Populism

Yoon’s announcement should be popular at home. Polling from last year shows 71 percent of South Koreans believe their military should acquire the bomb. Surveys from previous years have shown similar numbers, but mainstream politicians have largely stayed away from advocating proliferation.

Stephen Herzog
Stephen Herzog is a senior researcher in nuclear arms control at the Center for Security Studies of ETH Zurich, the Swiss Federal Institute of Technology, and an associate of the Harvard Kennedy School’s Project on Managing the Atom.

  

Yoon’s nuclear rhetoric also made headlines during the 2022 presidential election, when a nuclear South Korea became a mainstay of his conservative People Power Party’s platform. During the campaign, Yoon called for South Korea to host U.S. nuclear weapons before he then backtracked, saying this could violate the Nuclear Non-Proliferation Treaty (NPT).

Now, rapidly increasing North Korean missile testing, a growing Chinese arsenal, and the fallout from Russia’s invasion of Ukraine have worsened Seoul’s security environment.

Lauren Sukin
Lauren Sukin is an assistant professor in the Department of International Relations at the London School of Economics and Political Science (LSE) and a nonresident scholar in the Nuclear Policy Program at the Carnegie Endowment for International Peace.

These developments allow Yoon—the country’s first populist president—to brandish the national security card. Popular support for nuclear proliferation is, in part, borne of nationalist desires to increase South Korea’s independence from the United States. Research shows that nationalist attitudes and anti-American sentiment are prevalent among supporters of an indigenous nuclear arsenal. And nationalist leaders are more likely to seek the bomb.

Yoon also has good reason to believe his rhetoric will win domestic support. In addition to majority public support for a nuclear South Korea, his conservative base reacted enthusiastically to his hard line on Pyongyang during the campaign, including threats to preemptively strike North Korea’s nuclear weapons. This is a stark departure from the prior administration of Moon Jae-in, whose reconciliatory attitudes toward North Korea drew major criticism.

Seoul’s Security Environment

Nuclear populism may partially explain Yoon’s statement, but South Korea’s security environment is still a major factor. China’s nuclear forces are expanding, and North Korea launched more than ninety ballistic and cruise missiles last year. North Korean Supreme Leader Kim Jong-un also announced mass nuclear-capable missile production and revised his nuclear doctrine to allow preemptive strikes at early stages of crises. Observers believe Pyongyang is preparing its seventh nuclear test to add to the thermonuclear device the regime claimed it successfully detonated in 2017.

Seoul has relatively few alternatives to address its security challenges in this volatile environment. Sanctions are ineffective at compelling a nuclear reversal by Pyongyang, China and Russia are unlikely to pressure North Korea into renewed Six-Party Talks, and military exercises provoke North Korea and contribute to a security spiral. Tensions are rising between South Korea’s biggest ally, the United States, and biggest trade partner, China.

Proponents of a nuclear-armed South Korea also see lessons from the war in Ukraine. After the Soviet Union broke apart, Kyiv returned to Moscow the Soviet nuclear arsenal it had inherited in exchange for U.S., UK, and Russian security assurances. Despite disarming and joining the NPT in 1996, Ukraine was invaded by a state that had promised it security. Seoul has a more binding U.S. guarantee as well as a trip wire of 28,500 American troops stationed on its territory. Regardless, Moscow’s use of nuclear weapons to deter third-party intervention in Ukraine is a harrowing lesson. Fears grew that Russian behavior could embolden Pyongyang.

Despite these developments, Yoon was clear that for now Seoul won’t build a nuclear arsenal and will instead focus on strengthening its alliance with Washington. North Korea’s nuclear trajectory could, however, turn populist rhetoric about a South Korean bomb into a dangerous long-term reality.

Preventing this outcome falls to Washington, the traditional stalwart of the nonproliferation regime. It dissuaded South Korea from proliferating and coerced Seoul to join the NPT in 1975. If South Korea had nuclear weapons on its soil—indigenously developed or forward-deployed—dangers of nuclear escalation would increase markedly and costs to the nonproliferation regime would be enormous. The United States should convey the steep costs of proliferation while giving Seoul a greater voice in the alliance. Leveraging this combination of counterproliferation and assurance is the best bet to stymie a potential nuclear program.

Costly Nuclearization

To acquire its own nuclear weapons, South Korea would have to withdraw from the NPT. The 1968 treaty bans members other than Britain, China, France, Russia, and the United States from possessing nuclear arms. In exchange, the nuclear-haves promise to assist the have-nots with nuclear energy capabilities and to work toward disarmament. The treaty has played a major role preventing proliferation for more than five decades.

What had been fringe discussions over whether South Korea should leave the NPT or prioritize conventional contingency planning with the United States have become mainstream discourse as the security environment deteriorates. To date, North Korea is the only state to have withdrawn from the NPT, although nuclear-armed India, Israel, and Pakistan never joined. The treaty permits withdrawal in “extraordinary circumstances” if states provide a three-month notice, giving South Korea a theoretical exit strategy. But if every state with significant security concerns withdrew, preventing additional proliferation would prove impossible. Washington must emphasize that a precedent-setting NPT exit by Seoul is unacceptable.

After all, Seoul and Washington have recommended punishing states that withdraw by terminating all civilian nuclear cooperation. NPT withdrawers would then have to return materials and technology to their foreign suppliers. If South Korea were held to its own standard, the results would prove disastrous. The country relies on foreign nuclear fuel and energy cooperation with the United States and other partners. It expects 34.6 percent of its electricity to come from nuclear power by 2036. Leaving the NPT would likely mark the end of civilian nuclear cooperation—grinding these plans to a halt, severely harming South Korea’s economy, and costing the political leadership public support.

South Korean officials should also remember the international community punishes states that abrogate their nuclear responsibilities. The UN Security Council imposed an arms embargo, asset freezes, and travel bans following North Korea’s first test in 2006, and similar measures were briefly levied against India and Pakistan following their 1998 nuclear tests. Even if the United States shielded South Korea from UN sanctions, unilateral measures from actors like China, Japan, Russia, or the European Union—beyond a Nuclear Suppliers Group trade ban—would still bite. Broader understanding of these consequences could help steer South Korea away from the nuclear path.

North Korea and China also might go to extraordinary lengths to prevent a nuclear South Korea. Each has reacted harshly to Seoul’s military capability developments. Beijing imposed massive sanctions when South Korea acquired U.S. missile defenses. Pyongyang lashes out whenever South Korea runs military exercises or develops new capabilities. This should warn both Seoul and Washington about the potential risks of sending tactical nuclear weapons to South Korea. If South Korea built its own arsenal, the risks would be even greater. Deterring preemptive strikes by China and North Korea would be no small feat, to speak nothing of an arms race if Seoul successfully proliferated.

Moreover, nuclear weapons make crises more dangerous. As North Korea shifts to prioritizing nuclear use early in standoffs, decisionmaking pressures will skyrocket. Every time the regime thinks that a military exercise could be a surprise attack, that dual-capable aircraft flying near the border might launch a bolt from the blue nuclear strike, or that a U.S. soldier with an axe is invading rather than cutting down a poplar tree, a nuclear restraint decision has to be made. Nuclear arms in South Korea—indigenous or U.S.-deployed—would heighten pressure and shorten decision time lines.

Restructuring the Alliance

Although nuclear populist rhetoric can convey domestic political benefits to Yoon, actually following through would jeopardize South Korea’s security. There would be heavy costs for withdrawing from the NPT, attempting to proliferate, or possessing nuclear weapons. This isn’t to argue that South Korea wouldn’t succeed if it opted to proceed with nuclear weapons; threatened countries may make nearly inconceivable sacrifices to obtain the bomb.

South Korea faces dueling nuclear nightmares that place it between a rock and a hard place. The rock—entrapment—was demonstrated by the “fire and fury” rhetoric of former U.S. president Donald Trump when he threatened to annihilate North Korea without consulting allies. The hard place—abandonment—is highlighted by polling indicating Americans wouldn’t back nuclear retaliation against Pyongyang, even if U.S. forces in South Korea were killed. While public opinion isn’t synonymous with White House policy, it’s unclear how the United States would respond to a nuclear attack or invasion. And if there were U.S. nuclear weapons in South Korea, decisions about their use would be made in Washington.

A call for nuclear weapons is, fundamentally, a call for South Korea to know it can protect itself. In the short term, this requires a two-pronged policy. On the one hand, the United States must assure South Korea it will redouble efforts to stabilize East Asia. On the other hand, the United States must unequivocally demonstrate that a South Korean decision to pursue nuclear weapons will not end well. Washington should privately remind Seoul that while its security commitment remains, it will not tolerate NPT withdrawal and would cease civilian nuclear cooperation and impose sanctions if South Korea sought the bomb. This may keep short-term costs high enough to stave off nuclear proliferation.

In the long run, the United States cannot count on South Korea being content to follow its lead. Regional security threats have grown too dire. Fortunately, the array of U.S. capabilities that can be used in South Korea’s defense are more than substantial. The unanswered question is what level of say Seoul will have in how they will be used.

The future of the U.S.–South Korea alliance must be an equal partnership, not one of patron and protégé. More routine bilateral nuclear consultations and specific commitments spelling out how the United States will defend the country are needed to address South Korea’s security concerns and prevent intensified nuclear populism. This would increase confidence in the nuclear umbrella by providing Seoul firmer assurances and building trust that in the unlikely event it ever came down to it, the nuclear button would never be pushed unless the countries’ presidents do it together.