Japanese Prime Minister Sanae Takaichi's recent remarks in parliament — suggesting that a Taiwan Strait conflict could trigger an "survival-threatening situation" for Japan -are not only a troubling extension of Japan's domestic security concept but also an erosion of key principles of international law. By framing a contingency in Taiwan as grounds for exercising collective self-defense, Japan attempts to extend its domestic security framework to matters that are purely China's internal affairs.

This places Japan on a collision course with core postwar norms that prohibit interference in the internal affairs of other countries and bar the use of threat or force.

The Taiwan question is an internal affair of China, a status that is not merely a political declaration but firmly rooted in several international documents and historical facts. The Cairo Declaration of 1943 and the Potsdam Proclamation of 1945 unequivocally stipulated that Taiwan, which was seized by Japan in 1895, should be restored to China. Japan's Instrument of Surrender in 1945 formally accepted these terms. Furthermore, the United Nations General Assembly Resolution 2758 in 1971 recognized the Government of the People's Republic of China as the sole legitimate representative of China to the United Nations and acknowledged Taiwan as an integral part of China.

Against this backdrop, Japan's attempt to categorize a potential development in the Taiwan Strait as an "survival-threatening situation" under its domestic security laws violates the principle of non-interference in the internal affairs of sovereign states. Article 2 of the UN Charter explicitly prohibits intervention "in matters which are essentially within the domestic jurisdiction of any state". As a UN member state, Japan is bound by this fundamental principle. Invoking a domestic legal concept to justify potential military intervention in China's internal matter contravenes this core tenet of international law.

Moreover, the suggestion of military action raises questions about Japan's adherence to the UN Charter's strictures on the use of force. Article 51 of the UN Charter clearly defines the right to self-defense as applicable "if an armed attack occurs against a Member of the UN". This right is strictly circumscribed and does not extend to preemptive intervention based on an "existential crisis" that has not involved a direct armed attack on Japan.

Any military action or threat of force by Japan in the Taiwan Strait, without a direct armed attack on its territory, would also violate Article 2 of the UN Charter, which prohibits "the threat or use of force against the territorial integrity or political independence of any state". The recent deployment of missile units on the Yonaguni Island, merely 110 kilometers from Taiwan, further exacerbates these concerns. It is a provocative posture that risks miscalculation.

The main legal defect in Japan's position is the attempt to elevate domestic legislation above international commitments. This contravenes the Vienna Convention on the Law of Treaties, whose Article 27 states unambiguously that "a state may not invoke the provisions of its internal law as justification for its failure to perform a treaty".

International legal obligations — especially those derived from the UN Charter and from postwar settlements to which Japan explicitly subscribed — cannot be displaced or diluted by domestic legal reinterpretations. Japan's "existential crisis" framework is a creature of its domestic constitutional debate and security legislation. It has no grounding in international law and cannot be used to reshape or reinterpret obligations that arise under the UN Charter or customary international law.

In this sense, Japan's invocation of a domestic law for actions that affect another state's internal affairs is not only a legal misstep but a challenge to the international rule of law itself.

If states could adjust their international obligations simply by redefining their domestic legal terms, the global legal order would collapse.

Japan's stance on the Taiwan question also runs counter to the postwar settlement that Japan accepted as a defeated power. The Potsdam Proclamation, the Instrument of Surrender and subsequent diplomatic documents formed the legal and political foundation upon which Japan re-entered the international community. They also shaped the recognition, later reiterated in Sino-Japanese political agreements, that there is only one China and that the PRC is its sole legitimate government.

Takaichi's suggestion that Japan "cannot determine" Taiwan's legal status departs from these commitments and seeks to reframe an issue settled in Japan's own postwar obligations as a matter of contemporary political choice. Such a move not only undermines historical clarity but also risks destabilizing peace in East Asia.

Japan's move is a troubling departure from established international law because it seeks to circumvent the strict limits of the UN Charter, erodes the principle of non-interference, and challenges the hierarchy that places international law above domestic legislation. International law exists to restrain precisely such unilateral expansions of power.

The author is an associate professor and the head of the East Asia Marine Policy Project at the Center for Japanese Studies of Shanghai Jiao Tong University in Shanghai.

The views don't necessarily reflect those of China Daily.