The US has a new nuclear proliferation problem: South Korea

Last week, Seoul officially put its nuclear option on the table, for the first time since 1991. South Korean President Yoon Suk-yeol declared the country would consider building its own arsenal of nuclear weapons if the threat it faces from nuclear-armed North Korea continues to grow.

It will.

North Korea launched over 90 missiles in 2022. Those tests accompanied a major revision in North Korea’s nuclear strategy, which now allows the preemptive use of nuclear weapons in the early stages of a crisis. Experts expect North Korea’s ramped-up nuclear aggression will continue into the new year. Many even expect Pyongyang to conduct a new nuclear test, which would be the country’s first since 2017 and a watershed event against a backdrop of global turmoil.

South Korea faces strong strategic reasons to continue developing its own nuclear arsenal. While the United States has tried to keep a lid on South Korea’s nuclear ambitions, few traditional nonproliferation or counterproliferation policies are well-poised to reverse the current nuclearization of the North. It’s time for a new approach.

South Korean nuclear ambitions. South Korea faces an increasingly capable nuclear adversary in its northern neighbor. North Korea’s nuclear arsenal, first tested in 2006, has grown rapidly. The country now hosts dozens of nuclear weapons and continues to diversify its arsenal, building more sophisticated delivery capabilities, which include intercontinental missiles capable of reaching the United States. North Korea makes dozens of threats (usually against the United States) every month, many of them nuclear in nature. North Korea has been exceptionally belligerent lately, testing more nuclear-capable missiles in the past year than it did in the previous five years combined.

South Korea has a complicated relationship with its western neighbor, too. South Korea relies heavily on China for trade, but Seoul’s strong military alliance with the United States contributes to Chinese views of encirclement. So far, South Korea has walked a tightrope between its biggest military partner and biggest trade partner. But that won’t last. Most South Koreans consider that China will be their country’s biggest threat in the next 10 years.

South Korea has a troubled security environment, and the US security guarantee to South Korea is intended to make sure those threats don’t materialize. The guarantee offers reassurance that Seoul will be protected against any adversary. The guarantee is one of the United States’ strongest. The two countries boast significant military cooperation. The US military currently stations approximately 28,500 servicemembers in South Korea, regularly participates in large-scale military exercises with South Korean forces, and, under current policy, would fight under joint command with South Korean forces if a war were to break out.

But even with all this, the security guarantee doesn’t seem to be enough to keep down the bubble of proliferation advocates. Policymakers in South Korea have long called for a return of US tactical nuclear weapons, and a handful of more conservative politicians have occasionally suggested that the state would be better off with its own nuclear arsenal. Increasingly, this conversation has gone mainstream. The debate was even a key talking point and part of the conservative party platform in the last South Korean presidential election.

For years now, most South Koreans have supported the idea of the country building its own nuclear weapons. By 2022, such support had grown to over 70 percent. Russia’s continued use of nuclear threats in the Ukraine war may bring that number even higher, as nuclear anxiety grows. South Koreans are keenly aware that the United States and its allies have been effectively deterred by Russia’s nuclear arsenal, and they worry that a similar situation could repeat itself in Asia. Public support for South Korea building its own nuclear weapons has no doubt contributed to the policy’s rise out of the fringe and into the spotlight.

Is the US security guarantee enough? If South Korea is so concerned about nuclear threats from North Korea, a solution is to get reassurance that the United States will come to its aid in a fight against Pyongyang—or so the logic goes. But it isn’t that simple.

The United States and South Korea already have a tight-knit relationship, and faith in the US security guarantee is already high: At least 6 in 10 South Koreans are confident that the United States will fight with them against North Korea, if need be.

US politicians have regularly emphasized the criticality of the US-South Korean relationship, and the recent Biden administration’s Nuclear Posture Review made some usually heavy-handed promises in South Korea’s defense, even stating that “any nuclear attack by North Korea against the United States or its Allies and partners is unacceptable and will result in the end of that regime. There is no scenario in which the Kim regime could employ nuclear weapons and survive.”

But perhaps, a very credible security guarantee is just not enough—or perhaps it is even part of the problem. My research finds that, even when South Koreans have faith in the US alliance, many still don’t see it as a reliable solution to their perceived nuclear risks. In surveys, the more South Koreans believe the United States would use its nuclear weapons to defend them, the more they shy away from the US alliance and prefer that their own government build independent nuclear capabilities.

Although counterintuitive at first sight, the rationale is simple: Why would South Koreans trust the United States to be adequately cautious with its nuclear weapons—refraining from using them unless absolutely necessary? After all, the previous US president promised to rain down “fire and fury” on the Peninsula.

South Koreans have significantly higher levels of trust in their own government’s ability to make responsible nuclear choices than they do in an ally. Moreover, most South Koreans believe that their continued alliance with the United States will end up dragging Seoul into a nuclear war it otherwise could have avoided.

And understandably, South Koreans don’t want a nuclear war.

Any nuclear use on the Korean Peninsula—even if only North Korea were targeted—would likely have devasting environmental and health effects throughout the Peninsula. And Seoul is less than 200 kilometers (124 miles) from Pyongyang. Even in the event that North Korea invaded South Korea, most South Koreans still say in polls that they would prefer not to use nuclear weapons unless North Korea had already used them first.

Logically, South Koreans can’t take it for granted that this preference will be reflected in US policy. The US nuclear doctrine makes it clear that the United States carves out the right to “nuclear first use,” a tactic that involves launching nuclear weapons at an opponent before they have the chance to launch their own. Given that North Korea’s missiles can now reach the US homeland, any warfighting strategy for the United States is likely to prioritize destroying these assets—and a first strike would be the easiest way to accomplish that goal. For this reason, a credible US nuclear security guarantee alone won’t alleviate South Korea’s nuclear anxieties.

Build or borrow? President Yoon was quick to note that, even now, South Korea has options other than building its own nuclear arsenal. One of these is to request that the United States re-deploy some of its tactical nuclear weapons in South Korea. The United States withdrew its South Korea-based arsenal of approximately 100 nuclear weapons in 1991 to move past the Cold War. No US nuclear weapons have been stationed in the country since.

The re-deployment of these weapons, however, would do little to resolve the core issues of the current crisis—and maybe quite the opposite. Deployed US nuclear weapons in South Korea would heighten North Korea’s fears that the United States is preparing for the decapitation strategy it so boldly announced in its recent National Defense Strategy. There is also a moral hazard: Having nearby US nuclear weapons may embolden some in South Korea to push back harder against North Korea’s threats, making tensions even worse.

Moreover, unless these weapons were operated under South Korean command—a contingency that is extremely unlikely—issues around transparency, cooperation, and trust in US nuclear planning would still remain.

Re-deploying nuclear weapons would certainly be a signal of US interest in defending South Korea, but what’s needed now is a combination of commitment and caution. Forthright communication about when and why nuclear weapons would be used, combined with clear indicators about how nuclear use will be avoided is more important for the United States than simply showing it has the muscles. Those have been on display for decades already.

Redeployment of US tactical nuclear weapons would also leave South Korea vulnerable to many of the same risks as they would incur by building their own arsenal. In this sense, even opting for US redeployment over nuclear proliferation—although it may put less strain on the alliance in the short term—remains dangerous.

The redeployment of tactical nuclear weapons would not resolve the domestic political pressures at play in South Korea. Polling from the Chicago Council on Global Affairs finds that two-thirds of South Koreans would prefer for their government to build its own nuclear weapons than to accept the re-deployment of US tactical nuclear weapons, while below 10 percent prefer US weapons over South Korean ones. Outright opposition to US tactical nuclear weapons is also strong—at 40 percent, compared to just 26 percent opposed to South Korea building its own nuclear weapons. These figures suggest that a different strategy is called for, one that recognizes the need for more South Korean agency in the nuclear planning process.

Can stopgaps succeed? If neither cementing the guarantee nor redeploying tactical nuclear weapons is the answer, what can the United States do instead? One option can be to fight back against South Korea’s urge to build nuclear weapons with tried-and-tested nonproliferation policies. Nonproliferation leverages both carrots—security guarantees intended to protect a vulnerable country from nuclear threats—and sticks—sanctions and other punishments intended to dissuade this country from building nuclear weapons. Understandably, the US approach with its allies generally prioritizes carrots, but that may not continue to work with South Korea.

Could, therefore, counterproliferation strategies succeed?

Well, they did in the 1970s. When former South Korean President Park Chung-Hee embarked on a covert nuclear weapons acquisition program, the United States responded by threatening to scale back its support for South Korea and to reduce its military presence there. The pressure from Washington was a key component of Park’s decision to end the program—although domestic politics and concerns about the country’s international reputation also contributed to that decision.

But what worked in the past may not work today. In the 1970s, South Korea didn’t face nuclear threats as obvious as those it faces in 2023. The withdrawal now of US forces would be much more likely to convince Seoul that the only way to stop North Korea is to deter Pyongyang on its own.

Studies of South Korean public opinion show that support for nuclear proliferation remains relatively high, even when it is well understood by the public that building nuclear weapons would have a significant cost to the quality of South Korea’s alliance with the United States. Threatening to walk away, then, might just leave the United States looking regretfully over its shoulder.

Other counterproliferation policies have had mixed results. Experts argue that the threat of sanctions can often dissuade countries not to pursue nuclear weapons. However, once sanctions are imposed, they do little to reverse existing programs. Instead, targeted countries adapt, and the isolation that sanctions produce can cement the perceived need for stronger, more independent military forces.

South Korea may already be past the point at which sanctions would be useful. Multiple studies have found that South Koreans who support nuclear proliferation are not deterred by the threat of sanctions. Instead, South Koreans already anticipate that proliferation would result in significant sanctions—yet they would support the policy anyway.

The expectation that proliferation would result in sanctions is probably correct. A South Korean nuclear weapons program would almost certainly violate the obligations to nuclear nonproliferation and the peaceful, civilian use of transferred nuclear technologies that Seoul agreed to when it signed a nuclear cooperation agreement with the United States. This agreement, which remains in force until 2040, currently bans uranium enrichment in South Korea, at least without prior approval, as well as some types of plutonium reprocessing. Those capabilities would be needed for a robust nuclear weapons program. Violating its nuclear cooperation agreement with the United States could therefore trigger sanctions against Seoul. It would even legally enable the United States to demand that technology transferred under the agreement be returned. This is unlikely to be sufficient to stop a South Korean nuclear program if Seoul committed to one, but it does emphasize that the United States—if it so chose—could levy very heavy costs.

The United States can also advance nonproliferation through leading by example. Making it clear to South Korea that the global nonproliferation regime is critical—and that a South Korean withdrawal from the Non-Proliferation Treaty would be unacceptable—could help dissuade Seoul. After all, the country is highly concerned with its hard-earned international reputation, and unilaterally leaving a major international treaty would be no small step.

The United States can also commit itself to policies that prioritize restraint and arms control. Demonstrating its ability to embrace a more cautious attitude towards the use of nuclear weapons may diminish some of the concerns about Washington’s willingness to escalate to nuclear use, and it would model valuable norms in the nuclear space—norms that could perhaps even help balance against the behavior of other nuclear countries.