Japanese Defense Minister Shinjiro Koizumi announced on Friday the historic first delivery of US-made Tomahawk cruise missiles and Norwegian-developed Joint Strike Missiles to Japan's Self-Defense Forces.

The move marks a notable shift in Japan's military build-up, and its strategic implications are plain to see. What is far less clear, and far more legally questionable, is the argument Tokyo has been increasingly advancing in tandem with such military moves: that Japan could justify military intervention in a Taiwan Strait crisis under the banner of collective self-defense, particularly if the United States is involved.

That proposition has gained political traction. Last November, Japanese Prime Minister Sanae Takaichi suggested that a "Taiwan contingency" might constitute a "survival-threatening situation" under Japan's security legislation. Such a determination could permit the exercise of collective self-defense. On Jan 26, she went further, stating that if US forces were attacked during a Taiwan Strait crisis and Japan took no action, the alliance relationship could not be maintained.

The implication is clear. If conflict erupted and the US became involved, Japan would be both politically compelled and legally entitled to intervene in a "Taiwan contingency".

Yet this argument rests on a fundamental misreading of international law. Alliance politics, however compelling in strategic terms, cannot rewrite the legal framework established by the United Nations Charter. Under that, collective self-defense is not a discretionary tool activated by alliance solidarity or shared threat perceptions but a narrowly circumscribed, derivative right, anchored to the existence of a legally recognized victim state and the objective fact of an armed attack. So, the notion that the US-Japan alliance could serve as a legal "green light" for Japanese military intervention in the Taiwan Strait is untenable.

The flaw lies in how Article 51 of the UN Charter is often invoked but rarely read with care. The provision does not confer a general license to use force whenever allies perceive their security interests to be at stake. Rather, it preserves an exceptional right: the right of individual or collective self-defense if and only if an armed attack occurs against a state. Collective self-defense, in this structure, is not an independent entitlement but an extension of the victim state's own right to defend itself, exercised with external assistance.

This position has been consistently affirmed by the International Court of Justice. In its landmark 1986 judgment in Nicaragua v. United States, the Court rejected the proposition that alliance treaties could themselves generate a right to use force. The legality of collective self-defense, it held, is entirely derivative: it depends on a prior armed attack against a state that identifies itself as the victim and requests assistance. Without these elements, third states cannot invoke Article 51, regardless of alliance commitments or strategic affinities.

The Court reinforced this logic nearly two decades later in Armed Activities on the Territory of the Congo (DR Congo v. Uganda). There, it made clear that security anxieties, regional instability or anticipatory threat assessments cannot substitute for an objective armed attack. This insistence on a high threshold is not doctrinal rigidity but a safeguard against the institutionalization of war through alliances, where shared narratives of danger and risk become justifications for force.

Applied to the Taiwan question, these principles expose a legal impasse. For collective self-defense to be lawfully invoked, there must first exist a "victim state" capable of exercising individual self-defense. Taiwan, however, does not meet this criterion under the post-war international legal order. The Cairo Declaration of 1943 and the Potsdam Proclamation of 1945 stipulated that Taiwan, as territory taken by Japan, was to be returned to China — terms Japan formally accepted in its Instrument of Surrender. This legal continuity was later reflected in UN General Assembly Resolution 2758, which recognized the government of the People's Republic of China as the sole legitimate representative of China.

Within this framework, Taiwan cannot be treated as a sovereign state for the purposes of Article 51. It therefore lacks the legal capacity to exercise individual self-defense against the state to which it belongs, or to issue a valid request for collective self-defense to foreign powers. Without a legally recognized victim, the legal trigger for collective self-defense simply does not exist.

Some in Tokyo seek to bypass this obstacle by advancing a secondary argument: even if Taiwan itself cannot ground collective self-defense, Japan could intervene to assist US forces if they come under attack while operating in the Strait. This "referral" logic fares no better under international law. Collective self-defense cannot be transmitted through an intermediary. Its source must be the original victim of an armed attack, not a third state that has voluntarily inserted itself into a conflict.

If US forces were to intervene militarily in what is China's internal affair without Security Council authorization, their legal position would itself be contested. In such circumstances, the US would not qualify as a victim exercising self-defense, but could instead be seen as a party to an international dispute of its own making. Any Japanese military support under these conditions would not constitute collective self-defense but would amount to assistance in an internationally wrongful act, with corresponding legal responsibility.

The restrictive design of the UN Charter is deliberate. Its drafters, mindful of the catastrophic alliance-driven wars in the first half of the 20th century, sought to ensure that collective security would not be undermined by the very alliances meant to preserve it. Article 103 of the Charter underscores this intent by establishing the primacy of Charter obligations over all other international agreements. Alliance commitments may shape political expectations, but they cannot override the prohibition on the use of force enshrined in Article 2(4).

Ultimately, collective self-defense is a remedial mechanism, not an instrument of geopolitical engineering. It exists to respond to armed attacks against states, not to legitimize the extension of military conflict through alliance logic. The US-Japan alliance may be central to regional strategy, but it cannot alter Taiwan's legal status or manufacture a legal basis for the use of force where the Charter withholds one.

If strategic anxiety is allowed to replace legal thresholds, the right of self-defense will cease to function as a narrowly tailored exception and instead erode the prohibition on force. Upholding the discipline of the UN Charter is therefore not a matter of legal formalism, but a practical necessity for regional and global stability. In the absence of an armed attack, a recognized victim state and a valid request for assistance, alliance politics cannot supply what international law denies — and should not be permitted to try.

Zheng Zhihua is an associate professor and the head of the East Asia Marine Policy Project at the Centre for Japanese Studies at Shanghai Jiao Tong University in Shanghai.

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