Editor's note: Kong Qingjiang, a special commentator for CGTN, is the vice dean of the School of International Law, China University of Political Science and Law. The article reflects the author's opinions and not necessarily the views of CGTN.

On December 18, a senior official responsible for providing security policy advice to Prime Minister Sanae Takaichi claimed that given the increasingly severe security environment surrounding Japan, "Japan should possess nuclear weapons."

From the perspective of international law, if Japan develops or introduces nuclear weapons, it will undermine the legal foundation of the post-war international order.

Japan's possible nuclearization would pose fundamental challenges to the international nuclear non-proliferation system built on the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), the cornerstone treaty under which non-nuclear-weapon states commit not to acquire nuclear weapons; and nuclear-weapon states commit to nuclear disarmament, while safeguarding the right of all countries to peacefully use nuclear energy.

The effectiveness of the NPT relies on the clear legal distinction between "nuclear-weapon" and "non-nuclear-weapon" states. As a non-nuclear-weapon state party to the NPT, Japan's core legal obligation is "not to manufacture or otherwise acquire nuclear weapons or other nuclear explosive devices."

Not only will the act of developing or introducing nuclear weapons by Japan directly undermine the NPT treaty system, but allowing other countries, for example, the United States, to deploy nuclear weapons on its territory (a practice called "nuclear sharing"), will constitute serious violation of this fundamental obligation of the NPT treaty. It will blur the boundary between these two categories of states, weaken the normative effect of the treaty, set a dangerous precedent for other non-nuclear-weapon states, and may lead to a sharp increase in the risk of nuclear proliferation.

Ironically, on the one hand, Japan has time and again publicized its Three Non-Nuclear Principles ("not possessing, not producing and not permitting the introduction of nuclear weapons" into Japanese territory), seemingly to assure the international community. The Three Non-Nuclear Principles were even formally adopted by the Diet, the Japanese Parliament, and successive Japanese governments from time to time reminded the international community of the principles.

Due to the doctrine of promissory estoppel, a legal principle "to ensure a party does not go back on their promise when another party has relied upon that promise," under international law, the Three Non-Nuclear Principles have acquired the nature of international commitments. Abandoning these principles will be seen by the international community as a violation of Japan's international commitments that will seriously damage its national credibility.

On the other hand, Japan has long portrayed itself as the "only country to have suffered atomic bombings" and a "champion of nuclear non-proliferation." If it turns out to be a possessor of nuclear weapons, it will severely damage the moral authority and universality of the NPT, even trigger a chain reaction, thus undermining global strategic stability.

Japan's possible nuclearization would also violate a series of post-war international legal documents, which form the legal cornerstone of the post-war general international order.

For example, the Cairo Declaration and the Potsdam Proclamation, which dictate Japan's obligation of peaceful development as the defeated country in the Second World War, explicitly require Japan to completely disarm militarism. Seeking to acquire nuclear weapons, which stand at the top of the weapons of mass destruction, runs counter to the spirit of thorough demilitarization established by these documents.

In addition, Japan's possible nuclearization will have a damaging effect on other international nuclear disarmament and security norms. Take the Comprehensive Nuclear-Test-Ban Treaty (CTBT) as an example.

Japan was one of the first countries to sign and ratify the treaty, which aims to comprehensively ban nuclear test explosions and promote nuclear disarmament. If Japan were to move toward nuclear armament, it would have to embark on nuclear testing, which would seriously contravene the purpose and objectives of the CTBT.

Japan's nuclearization ambition, though not yet expressed officially, is particularly alarming, given that it has reportedly long stored, in the name of developing nuclear energy, up to 47 tons of plutonium, enough to manufacture thousands of nuclear bombs. Its plutonium stockpile far exceeds civilian needs, and the international community has already raised serious doubts about whether Japan's stockpiling of nuclear materials is in line with the "peaceful purpose" obligation under the NPT treaty.

Any hint that Japan might embrace a nuclear armament program should be scrutinized against this backdrop. After all, if civilian nuclear materials were diverted for military purposes, it would constitute the most fundamental violation of Article IV of the NPT, which stresses that nuclear energy should be developed, researched, and used only for peaceful purposes.

In summary, from the perspective of international law, any attempt by Japan to seek nuclear weapons is a systematic challenge to the entire post-war international legal order established under the Cairo Declaration and Potsdam Proclamation, and an overturn of the NPT system.

It would not only constitute serious violation of Japan's obligations under the core international legal instruments, but also amount to abuse of trust in the peaceful use of nuclear energy, which is the basis of the NPT system.

Such an attempt, which will potentially drag the region, and even the world, into the dangerous situation of a new nuclear arms race, should not be viewed as an isolated adjustment of defense policy. The international community, which has sufficient legal grounds, should instead take resolute measures to stop it.

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