North Korea’s nuclear-weapon developments and belligerent rhetoric, along with China’s military modernization and growing assertiveness, are creating direct challenges for Japan and South Korea, Washington’s Northeast Asian allies. In response, the United States has adapted its force posture and declaratory policy, and taken important steps to strengthen deterrence and reassure its allies. The recent decision to send an additional US Army combat force of eight hundred soldiers to South Korea with tanks and armored troop carriers and the pledge to maintain the US nuclear umbrella against North Korean threats is another step in Washington’s efforts to enhance defense of its ally.

While major conflicts have been deterred, it is unclear whether Japan and South Korea are reassured. Publicly and in private discussions, Japanese and South Korean officials insist that they trust US defense commitments. But they ask revealing questions about the conditions under which the United States would act, and how it would do so. They wonder about their roles and responsibilities, as Washington presses them to assume more of the defense and deterrence burden. And they worry about the reduction of roles and numbers of nuclear weapons in US strategy and, despite Washington’s rebalance to Asia, the ability of the United States to defend them well in a fiscally constrained environment. Plainly, US disengagement is a concern.

Could these concerns drive Japan and South Korea to resort to self-help and develop nuclear weapons? Both are technologically capable of going nuclear quickly, and this would be the cheapest way of increasing their indigenous military capabilities. But the real question is whether they would be willing to do so. While Japan remains allergic to the idea of crossing the nuclear threshold, there is growing public support, backed by influential elites, for manufacture of nuclear weapons in South Korea.

Neither Japan nor South Korea would develop nuclear weapons lightly. In all probability, the determining factor in their decision would hinge on its impact on their alliance with the United States.

What reaction, then, should they expect from Washington? There are two alternatives. One is that while unhappy, the United States would keep its alliances to maintain a favorable balance of power in East Asia. That logic would be bolstered by the idea that possession of nuclear weapons by each country would strengthen deterrence of North Korea and China. US policymakers would view Japan and South Korea as the United Kingdom―a US ally with nuclear forces integrated with US forces, with shared nuclear roles and responsibilities―or France―an ally operating independent nuclear forces. In other words, geopolitical dimensions would dominate the US reaction.

The second alternative is that the United States would terminate its alliances. Washington would conclude that permitting a nuclear-armed Japan and South Korea to remain as allies would drive others to follow suit. It would assess that the odds of this happening in Asia are high given growing nuclear latency, complex regional dynamics, and the absence of an organization like the North Atlantic Treaty Organization to hold US allies and partners together. Washington would also fear that this cascade could spill over into other regions, threatening the entire nonproliferation regime, creating instability, increasing war prospects, and ultimately eclipsing the US role as a responsible stakeholder for international order. Here, nonproliferation considerations would drive the US reaction.

Should Washington choose geopolitics over nonproliferation? The short answer is no. In the face of Japan’s and South Korea’s nuclearization, the United States should cut them adrift because endorsing their decision (through a UK-like arrangement) or acquiescing to it (à la France) would be untenable.

Recall that the United Kingdom and France went nuclear before the conclusion of the Nuclear Nonproliferation Treaty (NPT). (Even then, this generated important concerns in Washington.) With nonproliferation now proscribed under international law, entrenched as an international norm, and given such a major focus in US nuclear policy, not upholding it for its allies would be a nonstarter. Significantly, the net result would be a double failure for Washington: a reassurance failure and a failure to enforce nonproliferation rules, exposing the United States to considerable risks it should not take.

The 2010 US Nuclear Posture Review commits the United States to strengthening regional security architectures as needed and in a tailored manner. In Northeast Asia, substantial progress has been achieved through the bilateral consultative mechanisms it has established with Japan and South Korea. As the “strengthening” process continues, Washington should remind its allies that if they broke out of the NPT, they would break up their alliance.

David Santoro is a senior fellow at the Pacific Forum CSIS, where he specializes in nonproliferation and nuclear security, disarmament, arms control, and deterrence issues, with a regional focus on the Asia-Pacific region and Europe. You can follow him on Twitter: @DavidSantoro1.