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Opinion Should South Korea go nuclear? That’s a decision for Seoul, not Washington.

President Biden and South Korean President Yoon Suk Yeol in 2022. (Manuel Balce Ceneta/Associated Press)

In early January, President Yoon Suk Yeol made news by suggesting that, with the North Korean nuclear threat rising, South Korea might want to build its own nuclear arsenal. After a domestic and foreign backlash, Yoon, who arrives in Washington this week, walked back that suggestion. But in March, Seoul Mayor Oh Se-hoon — a prominent member of Yoon’s own party who is seen as a leading presidential candidate in 2027 — also raised the possibility of South Korea going nuclear. That option was backed, in a recent poll, by 77 percent of South Koreans.

Popular support for a South Korean nuclear deterrent, while strongly opposed by the Biden administration, is hardly surprising given the rapid expansion of the North Korean nuclear program. Any hopes that Pyongyang might give up or even freeze its nuclear stockpile were dashed by the failure of President Donald Trump’s talks with North Korean leader Kim Jong Un in 2018 and 2019. Since then, the North has raced ahead with capabilities ranging from intercontinental ballistic missiles that can hit the United States to tactical nuclear weapons that can saturate South Korea.

Every week seems to bring bloodcurdling new threats from Kim — the latest being the unveiling of an underwater drone supposedly capable of carrying a nuclear weapon and unleashing a “radioactive tsunami.” Kim has not only declared that he will never give up his nuclear weapons, but also claimed the right to use them preemptively if his regime feels threatened.

South Koreans are understandably worried and wonder if they can still count on the United States to defend them if, by doing so, it would put U.S. cities at risk of nuclear annihilation. Koreans are concerned that their country could meet the same fate as Ukraine — another nonnuclear state attacked by a nuclear-armed neighbor.

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South Korean leaders have been discussing ways to strengthen “extended deterrence,” such as redeploying U.S. tactical nuclear weapons in South Korea (they were withdrawn in 1991); replicating the nuclear-sharing agreement the United States has with NATO allies (European aircraft can deliver U.S. nuclear warheads in wartime); or even developing a domestic nuclear weapons capability.

The U.S. position is that having nuclear weapons permanently stationed in South Korea — whether American or, potentially, South Korean — is dangerous and unnecessary, because the United States could always destroy North Korea with nuclear weapons fired by distant submarines, bombers and missiles. The administration has been trying to assuage Seoul’s concerns by promising greater consultation about the use of nuclear weapons and more frequent visits by U.S. nuclear-capable bombers and warships. This will undoubtedly be near the top of the agenda when Yoon comes to the White House for a summit on Wednesday.

A senior administration official told me that it “is very profoundly troubling for us” to hear South Koreans discuss the possibility of building their own nuclear weapons. “We stand by the principles of the Nuclear Non-Proliferation Treaty,” this official told me. “We don’t want to see the spread of nuclear weapons. If they spread, they won’t just spread to countries in which we have confidence.”

The U.S. position is perfectly understandable. But it’s also easy to see why many Koreans are not reassured by U.S. security guarantees, given that Trump flirted with pulling U.S. troops out of South Korea if Seoul didn’t dramatically increase the amount of money it paid to subsidize them.

What if Trump or a Trump mini-me wins the presidency in 2024? Could South Korea count on an America First president to risk nuclear conflagration on behalf of a distant ally? The senior administration official told me that “our fundamental view is that U.S. extended deterrence commitments to the Republic of Korea are rock solid,” but, of course, the current administration cannot bind a successor.

We need to think carefully about whether our anti-proliferation assumptions still hold in a world where nuclear threats are growing, U.S. military dominance is fading and domestic support for U.S. global leadership is declining. The best guide I have seen to the arguments for and against South Korean nuclear weapons is a forthcoming article in Lawrence Livermore National Laboratory’s Occasional Papers series by Brookings Institution senior fellow Robert Einhorn, who served as the State Department’s senior adviser on non-proliferation during the Obama administration. (Einhorn provided me with an advance copy and walked me through his arguments.)

The article lists 10 reasons it could make sense for South Korea to go nuclear. These include the prospect that “it would strengthen deterrence against North Korea,” “force North Korea to deal with the Seoul government more seriously,” enhance South Korea’s “image as a strong, independent, successful player on the world stage” and reduce the risk of a North Korean nuclear attack on the U.S. homeland.

While some of these contentions are arguable, there is little disputing Einhorn’s assumption that “South Korea would be a responsible nuclear-armed state.” Moreover, South Korea has a right to withdraw from the 1970 Non-Proliferation Treaty. Article X allows any signatory to leave if “extraordinary events … have jeopardized the supreme interests of its country.” The North Korean nuclear program certainly qualifies.

Having presented the “pro” case, Einhorn then goes on to list nine compelling arguments against South Korea building its own nukes. He argues that, with 28,500 U.S. military personnel based in South Korea and the two nations bound by a defense treaty, South Korea can trust the “extended deterrence” provided by “U.S. strategic assets” that are “off-shore and mostly out of sight.”

Other arguments against South Korea going nuclear include the possibility that doing so could weaken the U.S. alliance, with American politicians wondering: Why do we need to risk our own troops to defend a nuclear-armed ally? It could damage the global non-proliferation regime. And it could limit South Korea’s access to the imported uranium it needs to run its nuclear power industry, which generates 27 percent of the country’s electricity.

Einhorn’s conclusion: “Acquiring an indigenous nuclear weapons capability is not the answer to South Korean security concerns.” But some other U.S. experts have reached a different conclusion. “It’s a real dilemma for responsible South Koreans,” Einhorn told me. “President Yoon and his advisers are clearly weighing all their options.”

For the moment, Yoon has made clear that he isn’t pursuing nuclear weapons capability and would prefer enhanced U.S. deterrence. But if, in the future, South Korea does decide to go nuclear, it should not be a game changer for the United States. The United States has long tolerated nuclear weapons owned by friendly states such as France, Britain, Israel, Pakistan and India, while opposing their acquisition by rogue regimes such as Iran and North Korea. Having South Korea join the nuclear club wouldn’t change that.

Ultimately, it should be South Korea’s call. We should refrain from applying heavy-handed pressure and respect whatever decision our democratic ally makes. As Yoon and President Biden will affirm this week, both Washington and Seoul want the same thing: a secure and prosperous South Korea aligned with the West.