The alliance between the United States and South Korea seems as if it should be stronger than ever. America is finally shifting its strategic attention to Asia, aided by the end of its “forever war” in Afghanistan. Meanwhile, the growing North Korean nuclear threat should be energizing shared U.S. and South Korean interests in denuclearization and deterrence. At a glance, the bonds that lock Seoul and Washington together appear solid.

In fact, the alliance is in trouble — pulled apart by powerful geopolitical forces. The only way to save it might be for South Korea to move in a direction that much of Washington considers unthinkable: to develop an independent nuclear arsenal.

The Trump years certainly damaged the relationship; President Donald Trump made clear that he thought South Korea was taking advantage of the United States. But the true root of the problem lies in two long-term trends. First, the rise of China is creating a rift between American and South Korean foreign policy priorities. Managing the growth of China’s power has become America’s primary national security goal. As the costs and dangers of countering China rise, Washington increasingly expects its allies to join in this effort.

But the South Koreans never signed up for that deal. Their alliance with the United States has always been about North Korea. A counterbalancing effort against China would poison South Korea’s relations with its No. 1 trading partner — which is also the most powerful country in the region. Fear of offending China partly explains South Korea’s reluctance to join “the Quad,” a U.S.-led alignment that includes India, Australia and Japan. The United States is an important player in East Asia right now; China, Koreans know, will be their neighbor forever.

The situation is made worse by a second trend: the growing sophistication of North Korea’s nuclear capabilities. Pyongyang has made major strides toward developing high-yield thermonuclear weapons and missiles that can carry them to the continental United States. This development fundamentally changes the alliance’s risk-reward calculus. For decades, American leaders accepted that defending South Korea could be very costly, possibly claiming the lives of thousands of U.♠S. soldiers. But now the costs of a conflict in Korea could be truly catastrophic for the United States.

In the event of war, leaders in Pyongyang would have powerful incentives to use nuclear weapons to stalemate South Korea’s conventional military superiority. Should the United States retaliate, the American homeland would become a target. War on the Korean Peninsula could thus lead to the destruction of multiple American cities — and the political, economic and social chaos that would follow. The American people never signed up for that deal.

As a result the alliance faces credibility problems. South Korea can’t be sure it can depend on its U.S. ally for protection. At the very moment that the two countries’ strategic priorities are diverging, the risks that the United States must bear to defend South Korea are growing a thousand-fold. North Korea, too, may question whether Washington would rush to Seoul’s aid during a war when doing so would threaten the survival of the United States.

Washington and its allies faced a similar credibility problem during the Cold War. In the early 1950s, NATO members wondered whether the emerging Soviet nuclear threat to the U.S. homeland meant they could no longer rely on the United States. Would the Americans really sacrifice Boston to protect Bonn? The allies addressed this credibility problem with three partial solutions. Britain and France acquired their own nuclear arsenals. For others, NATO implemented nuclear sharing: storing some U.S. weapons on allied bases in Europe, to be transferred to the allies if war erupted. And the U.S. military stationed large ground and air forces on the continent, deploying troops with their families, to intertwine the United States in any major war from the outset. 

The United States has shown no interest in creating a Korean nuclear sharing agreement — for good reason. An agreement premised on plans to give nuclear weapons in a time of war to nonnuclear allies is legally questionable, given that the Non-Proliferation Treaty (NPT) prohibits their transfer. (Indeed, NATO’s nuclear sharing exists in a legal gray area.)

Additionally, with modern locks, such weapons would still be firmly in the control of American leaders, and hence no more credible than other elements of the U.S. nuclear arsenal. Nor does the United States seem likely to increase the size of its conventional deployments on the Korean Peninsula or intertwine them with the Korean forces on the border. In fact, the number of American troops there has declined, with the forces positioned farther from the demilitarized zone.

That leaves the first option, however distasteful it may seem: South Korea may choose to acquire its own nuclear arsenal. Such a move would protect South Korea against the North Korean threat — more securely than today’s arrangement — and help the country manage its other long-term security problem: how to retain political independence in a region where China wields ever-greater power and influence.

Some analysts see nuclearization as a nonstarter, fearing it would make South Korea — an NPT member — a pariah like the North. But North Korea’s development of nuclear weapons was illegal, violating multiple United Nations Security Council resolutions. South Korea’s would be legal and justified. The NPT’s Article X was written for precisely the circumstances that South Korea faces today. It offers a withdrawal option if a member faces “extraordinary events” that “have jeopardized the supreme interests of its country.” North Korea’s illegal development of nuclear weapons and its threats against the South certainly qualify as extraordinary circumstances. South Korea’s development of nuclear weapons would be a proportional response to North Korea’s actions.

Seoul may already be heading in this direction. Former foreign minister Song Min-soon has said that South Korea “taking its own measures to create a nuclear balance on the peninsula” is an idea “widely touted” by leaders and analysts. Seventy percent of the South Korean public endorses the move. And Seoul’s new fleet of ballistic missile submarines is an unusual acquisition: a vastly expensive way to deliver a handful of conventional missiles. Those subs, however, would make an ideal platform for a future nuclear deterrent.

A South Korean nuclear arsenal is not what Washington prefers — indeed, it goes against a core U.S. policy of preventing nuclear spread. But it might be the best course given the weakened foundation of the alliance. If Seoul decides to take this step, the United States should focus blame where it belongs — on Pyongyang’s illegal nuclear program — and render political support to a valued ally.

Twitter: @profLind