Nuclear nonproliferation in East Asia looks, to some commentators, increasingly precarious. In recent months, North Korea restarted the Yongbyon reactor and conducted missile tests. South Korea tested a submarine-launched ballistic missile. China tested hypersonic missiles and constructed new missile silos. Japan continued to show interest in developing precision-strike capabilities. The new “AUKUS partnership” provides for the transfer of nuclear-powered submarine technology to Australia.

All point to a destabilizing trend. The recent U.S.-China summit between Presidents Joe Biden and Xi Jinping could lead to strategic stability talks, but even if such discussions take place, improving regional security will be a long and arduous road. With the security environment heating up, could international law play a role in cooling nuclear ambitions?

An Unravelling Regime?

The newest agreement on the nuclear scene is Treaty on the Prohibition of Nuclear Weapons (TPNW), which — as the name implies — bans nuclear weapons. Its effectiveness is limited, though: no states with nuclear arsenals and few under the nuclear umbrella have signed on. South Korea and Japan, for example, neither formally participated in TPNW negotiations nor signed the agreement.

Both are, however, members of the Nonproliferation Treaty (NPT), an agreement that has long been the cornerstone of the nuclear nonproliferation regime. The NPT’s “grand bargain” mandates that non-nuclear signatory states refrain from building nuclear weapons and submit to international safeguards on nuclear energy technology, in exchange for the nuclear-armed signatory states committing to eventual denuclearization.

However, a review conference intended for April 2020 has since been delayed several times, and scholars debate whether or to what degree the regime is unraveling. Both sides of the grand bargain have struggled to hold up their ends of the deal. Nuclear-armed states have largely failed to progress towards denuclearization, while some non-nuclear states, like Iran, stand accused of cheating on their NPT obligations. Some analysts now even argue that if states facing nuclear threats — like South Korea — withdrew from the NPT, it would be both “legal and justified.”

Nevertheless, there is cause for optimism. Even amid an increasingly hostile security environment, international law may yet provide meaningful safeguards against nuclear proliferation in East Asia and elsewhere.

Withdrawing from the NPT Wouldn’t Be Easy

Withdrawals from the NPT would violate a carefully crafted global precedent. North Korea is the only state to have withdrawn, in 2003. Article X of the NPT allows for withdrawal by a state party — after a three-month notice period — if “extraordinary events…have jeopardized the supreme interests of its country.”

In withdrawing, Pyongyang argued it faced “extraordinary events.” North Korea claimed U.S. hostility necessitated it develop a nuclear deterrent. Some have argued states like South Korea or Japan could, theoretically, use a similar logic to withdraw from the NPT. They could claim North Korea’s nuclear status as an “extraordinary event” justifying their own proliferation. But many states face nuclear threats; if simply having a nuclear adversary became a sufficient condition for invoking Article X, the already-at-risk NPT regime could be further endangered.

Withdrawals using this argument would also likely face legal challenges. Other states could argue that customary international law imposes a “good faith” standard. The requirement that a withdrawing state submit its reasons to the Security Council — a clause not included in other arms control treaties with the “supreme interests” rule, such as the Partial Test-Ban Treaty — suggests the Security Council has the right to evaluate a state’s interpretation of “extraordinary events” to ensure withdrawal is in good faith. Without a direct catalyst, a claim Seoul or Tokyo was newly threatened by North Korea’s nuclear status — or for that matter, that Iran was newly threatened by Israel or the United States — may be seen as suspect. The Security Council could consequently levy punishment.

North Korea should also not be a model for future withdrawals because its exit was legally questionable. North Korea did not comply with the NPT’s three-month notice requirement. Instead, North Korea declared its withdrawal would be effective immediately. (Other legal debates regarding North Korea’s nuclear status center on pre-withdrawal violations of the NPT, such as whether North Korea can still be subjected to punitive measures or allowed to retain acquired nuclear material from breaches prior to 2003.)

Lawful withdrawals should comply with the notice period. But the three-month probationary period poses a dilemma for states wanting to withdraw: It creates a window of opportunity for adversaries to take preventative action against new nuclear programs. Imagine, for example, if South Korea announced it wanted to build nuclear weapons. North Korea and China would have many reasons to take advantage of the notice period to interfere. China could impose bilateral sanctions. North Korea could target nuclear scientists. Either could engage in shows of force intended to slow or reverse South Korean proliferation.

Constraints Beyond the NPT

In addition to their NPT obligations, bilateral agreements and domestic laws could constrain proliferation in South Korea and Japan. For example, South Korea is a signatory to the “Joint Declaration of South and North Korea on the Denuclearization of the Korean Peninsula.” North Korea has undoubtedly violated the agreement, so South Korea could easily renounce the declaration. Doing so might be disadvantageous, though. The agreement provides a possible framework for an inter-Korean inspections regime — though it has never been used. Brazil and Argentina have similarly agreed on a bilateral inspections regime, which now works closely with the IAEA. South Korea’s continued participation in this declaration could provide Seoul with leverage in talks with Pyongyang.

South Korea and Japan could also face bilateral sanctions from the United States if they chose to develop nuclear weapons. Doing so would violate obligations to nuclear nonproliferation and the civilian use of nuclear infrastructure that enabled U.S. transfers of nuclear technologies and materials in nuclear cooperation agreements with the United States (known as “123 agreements” for the section of the U.S. Atomic Energy Act of 1954). For example, the U.S.-R.O.K. 123 agreement, which underwrites the nations’ nuclear energy trade, requires “assurance that all activities under the agreement will be for peaceful purposes only.” The agreement bans South Korea from uranium enrichment, and some types of plutonium reprocessing, without prior U.S. approval. South Korean violations would enable the United States to demand any transferred nuclear technologies or materials be returned. Failure to comply could lead to sanctions. The terms remain in force until 2040, after which the agreement can be renewed.

The U.S.-Japan 123 agreement is notable in that it allows Japan to engage in both enrichment and reprocessing. (By contrast, the so-called “gold standard” agreement between the United States and the United Arab Emirates bans both.) Nevertheless, the U.S.-Japan agreement does require that all technology transfers be used exclusively for peaceful purposes. It also contains provisions that could cause any Japanese detonation of a nuclear explosive to halt ongoing cooperation and to require transferred nuclear technology to be returned to the United States. The agreement operates in perpetuity, but either party can terminate it with a six-month notice.

Japan could face an additional legal barrier to nuclear proliferation in Article 9 of its constitution, which renounces the maintenance of “land, sea, and air forces, as well as other war potential.” Since 1965, Japan’s Cabinet Legislation Bureau has maintained Article 9 does not actually prohibit nuclear proliferation, so long as nuclear weapons are defensive in nature. However, this interpretation is contested, as is the assumption that nuclear weapons can be purely defensive. Some have argued the constitution presents a legal constraint to nuclear proliferation, or that it has contributed to widespread societal opposition to nuclear weapons. Japanese nuclear proliferation, then, would require some degree of “reinterpretation” of, if not outright amendment to, Article 9.

States can also make non-binding commitments to nonproliferation, as Iran did in signing the Joint Comprehensive Plan of Action (JCPOA). Per the terms of the agreement, if Iran were to fail to implement its nuclear commitments, the other JCPOA participants would be permitted to reimpose economic sanctions. However, this mutually reinforcing set of political commitments began to unravel following Trump’s unilateral withdrawal from the JCPOA in 2018. The United States subsequently re-imposed sanctions, despite continued Iranian compliance. Iran, in turn, ramped back up its nuclear program. (JCPOA participants are meeting again this week in the hopes of reviving the JCPOA or agreeing on a new arrangement.) The 1994 U.S.-North Korea Agreed Framework was a similarly quid-pro-quo-based approach to nonproliferation. It was designed to roll back the North Korean nuclear weapons program, though it did not, of course, succeed. Mixed track record aside, new bilateral or multilateral agreements could be an additional option if any new nuclear weapons programs were to come into play.

The Persistent Influence of International Law

Although the strength of the nuclear nonproliferation regime is perennially questioned, international law provides valuable infrastructure. Large multilateral agreements, like the NPT, as well as bilateral agreements, like 123 agreements, remain vital in the nonproliferation toolkit. Moving forward, the United States and others should continue to build and strengthen a diverse set of agreements to regulate nuclear technology, nuclear trade, and nuclear weapons.

Fortunately, despite the seemingly continuous stream of adverse security developments, neither South Korea nor Japan have an ongoing nuclear weapons programs, and any decision to create such programs would be enormously costly and legally dubious. These facts mean opportunities remain for the United States to more effectively manage the concerns of its allies and work towards arms control — and the maintenance of nuclear nonproliferation — in East Asia.

Image: A man watches a television report showing a news broadcast with file footage of a North Korean missile test, at a railway station in Seoul on October 19, 2021, after the South’s military said a North Korean weapons test was believed to be a submarine-launched ballistic missile. (Photo by ANTHONY WALLACE/AFP via Getty Images)