South Korea has long relied on the United States to keep the North Korean nuclear threat at bay. Pyongyang began taking fitful steps toward a nuclear weapon during the Cold War, tested its first bomb in 2006, and today regularly issues nuclear threats against its southern neighbor. Seoul, meanwhile, shelters under the American nuclear umbrella that came with the defense alliance it signed with Washington in 1953, just after an armistice effectively ended the Korean War. For decades, this arrangement provided South Korea sufficient security assurance. But today, that assurance appears increasingly fragile. The United States has lost credibility and is no longer seen as a reliable ally, whether in Asia, Europe or the Middle East. American foreign policy is increasingly perceived as weak, unreliable and biased.

South Korea’s problem is twofold. First, North Korea’s capabilities are growing. Pyongyang has developed an intercontinental ballistic missile, which raises doubts about whether the United States would honor its alliance commitment and fight for South Korea, because North Korea can now strike American cities with a nuclear weapon. Secondly, Donald Trump, who has severely criticized the U.S.-South Korea alliance in the past, has entered his second term as U.S. president. Under Trump, the likelihood of Washington intervening in a conflict on the Korean peninsula will diminish further. The USA is now more concerned about China’s military might, which will overtake the USA in a few years’ time. Under these conditions, some experts believed that the bomb is the best way to contain the threat from the North. That is the fundamental argument for South Koreans who want their country to develop its own nuclear weapons. It’s about the need to protect themselves from an aggressive northern neighbor that is already a nuclear power in all but name and whose leader Kim Jong Un has vowed an “exponential increase” in his arsenal.

The counter-argument, which has has long stopped Seoul from pursuing the bomb, lies in the likely consequences. Developing nukes would not only upset the country’s relationship with the United States, it would likely invite sanctions that could strangle Seoul’s access to nuclear power. And that is to say nothing of the regional arms race it would almost inevitably provoke. But which side of the argument South Koreans find themselves on appears to be changing. Ten years ago, calling for South Korean nuclear weapons was a fringe idea that garnered little serious coverage. Today it has become a mainstream discussion. Recent opinion polls show a majority of South Koreans support their country having its own nuclear weapons program; and a string of prominent academics who once shunned the idea have switched sides. In such a conflict between the two countries, Pyongyang would almost certainly threaten nuclear attacks on American targets to deter U.S. participation. U.S. bases in the Asia-Pacific, Guam, or Hawaii would be threatened first, and then the U.S. mainland. This raises the potential cost of American assistance to South Korea far higher than it has ever been. And it is likely enough to make the United States hesitate before getting involved. Consider the war in Ukraine, where Russian nuclear threats have successfully limited U.S. support for Kyiv. If Moscow’s threats worked against the alliance-friendly President Joe Biden, then Pyongyang’s will very likely restrain the nationalist, transactional Trump. South Korea would then be left to fend for itself. To close this glaring gap in its security, Seoul is now considering a step that, until recently, was discussed only on the country’s political fringe: building its own nuclear weapons. In South Korea, this proposal has gone mainstream. According to a Chicago Council on Global Affairs poll conducted in 2021, 71 percent of South Koreans support nuclearization, an increase from the 56 percent support the Seoul-based Asan Institute for Policy Studies found in a survey in 2010. Other polls by South Korean think tanks have found similarly decisive levels of public support. Political elites remain divided but are more sympathetic to the idea now than at any point in South Korean history.

Today, the biggest obstacle to South Korean nuclearization is not a domestic constituency but a foreign one: the United States. There is a deep, decades-old bipartisan opposition in Washington to nuclear proliferation, even among U.S. allies. In recent years, the Biden administration has tried to keep Seoul satisfied with statements reaffirming U.S. security commitments. American pressure is likely the primary reason South Korea still participates in the international Nuclear Nonproliferation Treaty (NPT), which forbids its nuclearization. The United States has learned to live with nuclear partners before, however. The United Kingdom and France were both U.S. allies when they conducted their first nuclear tests, in 1952 and 1960, respectively, and Washington retained close ties with Israel after it developed a nuclear program in the 1960s, despite U.S. entreaties. South Korea’s anxiety over the dependability of the U.S. nuclear umbrella is also nothing new; U.S. allies had similar worries during the Cold War, when the Soviet Union threatened nuclear strikes against the U.S. mainland. But Washington’s refusal to let Seoul act on its concerns now sets the allies up for an unnecessary confrontation.

American opponents of South Korean nuclearization exaggerate the policy’s downsides, underappreciate its benefits, and ignore the United States’ own liberal values that call for Washington to tolerate a democratic partner’s national security choices, even when it dislikes them. If Seoul took this step, it would not trigger the breakdown of the international nonproliferation regime, as critics fear. North Korea’s nuclear capabilities undermine U.S. deterrence, but a South Korean nuclear arsenal can help fill the gap. A nuclear South Korea would be more self-sufficient, reducing the potential harm if Trump draws back from U.S. alliances and calming Seoul’s obsessive anxiety about the U.S. nuclear commitment. With South Korea better able to handle the North Korean problem on its own, the United States could devote more attention to its top priority in East Asia—competition with China. But first, Washington needs to stop getting in its ally’s way and start letting Seoul make its own decisions. A South Korean decision to nuclearize could, on balance, be good not just for South Korea but also for the United States.

North Korea’s nuclear and missile capabilities have rapidly expanded since Kim Jong Un assumed leadership, in 2011. Four of the six nuclear tests Pyongyang has conducted have taken place under his leadership. Since 2017, North Korea has test-fired multiple intercontinental ballistic missiles that can hit the U.S. mainland, as well as many short- and medium-range missiles that could blanket South Korea. Pyongyang is pursuing further advances, too. It seeks to make its missiles hypersonic and its nuclear bombs smaller, perhaps with technological assistance from Russia to accelerate its progress. To improve the survivability of its nuclear forces, Pyongyang has announced its intention to put them on submarines. And it is integrating small nuclear weapons into its army, including frontline units. North Korean strategy reflects these capability improvements. Pyongyang now routinely, almost casually, threatens to nuke South Korea and the United States. In September 2022, it promulgated a law permitting the preemptive use of nuclear weapons in the early stages of a crisis. In a speech announcing the law, Kim stated that North Korea’s nuclear status is “irreversible” and that its nuclear weapons are not a “bargaining chip” it would trade away in negotiations. The current disparity between the North’s and the South’s nuclear capabilities destabilizes the Korean Peninsula. It encourages North Korea to bully South Korea into making concessions when crises between the two inevitably erupt.The uncertainty it generates in Seoul as to whether Washington would come to its defense in a conflict—and, in turn, Washington’s refusal to tighten its commitment to Seoul—paralyzes the alliance and opens the door to miscalculation. Inter-Korean nuclear parity would end this dangerous impasse, as Seoul would be able to deter Pyongyang without relying on questionable American guarantees. South Korean nuclearization need not cause a rupture with the United States unless Washington chooses to create one. As South Korea’s primary security partner and longtime political patron—and running neck and neck with China to be South Korea’s biggest export market—the United States wields an informal veto. It has already tried to dissuade Seoul, reassure it, and vaguely threaten it. None of this has worked. Trump’s return will only deepen Seoul’s nuclear interest; just two days after his reelection, South Korea’s largest daily newspaper ran an editorial suggesting that the country might need its own nuclear weapons. Washington’s moves have failed to resolve the core security problems that a South Korean program can redress: North Korea’s relentless march toward ever more powerful weapons of mass destruction and the United States’ unreliability in (and likely after) the Trump era. The United States itself would never tolerate the nuclear vulnerability South Korea now experiences. Rather than insisting that its ally remain imperiled, Washington should drop its barriers to Seoul’s finding its own way to security.

In conclusion, South Korea has no nuclear weapons and is under the protection of an hypothétic U.S. “nuclear umbrella,” which guarantees a devastating American response in the event of an attack on its ally. But some experts question the effectiveness of such a security commitment, saying the decision to use U.S. nuclear weapons lies with the U.S. president.