Four political documents between China and Japan form a coherent system of legal and political commitments that Japan must strictly observe with respect to the Taiwan question. These commitments are progressive in binding force, clear in content, and inescapable in effect.

Recently, Japanese Prime Minister Sanae Takaichi invoked the so-called “survival-threatening situation” during a parliamentary hearing, claiming that Japan’s Self-Defense Forces could exercise the right of collective self-defense in the event of a “Taiwan emergency.” In response, Chinese officials have repeatedly clarified that such statements “seriously violate international law” and have lodged solemn representations.

It should be noted that the core danger of Takaichi’s erroneous remarks does not lie in a generic display of a “tough stance,” but rather in her attempt to construct a framework of “legitimacy” for military intervention in Taiwan, thereby externalizing and militarizing the Taiwan question, which concerns China’s sovereignty and territorial integrity. This approach blatantly undermines the fundamental principles of the post-war international order established by the United Nations Charter and is extremely egregious.

Why is it said that Sanae Takaichi has violated international law?

Takaichi’s fallacies clash directly with the core principles established by the UN Charter. As the foundational treaty of the United Nations, the Charter codifies the basic norms governing relations among states and serves as the cornerstone of modern international law. Article 2, paragraph 4, explicitly stipulates that “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state.”

The Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States in accordance with the Charter of the United Nations, adopted by the UN General Assembly in 1970, further reaffirms that no state shall interfere in the internal affairs of another by means of force, threat, economic coercion, or otherwise, and opposes violations of a state’s sovereignty and territorial integrity in any form. Given that the one-China principle has long been recognized by UN General Assembly Resolution 2758 and by the international community, Japan’s discussion of deploying troops and exercising the right of collective self-defense under the pretext of a “Taiwan emergency” essentially constitutes a threat of force and interference in China’s internal affairs, thereby violating the fundamental principles of international law.

Takaichi’s attempt to pave the way for so-called “military intervention in Taiwan” through domestic law represents a severe reversal of legal logic. Her citations, such as the so-called “survival-threatening situation,” derive from Japan’s Legislation for Peace and Security enacted in 2015, which loosened restrictions on the overseas use of force through expansive interpretation. However, a basic tenet of international law is that no state may invoke its domestic law as justification for failing to fulfill its international obligations. Japan has long committed, through the UN Charter and political documents with China, to abide by the principles of refraining from the use of force and non-interference in internal affairs, and to recognize that “Taiwan is part of China.” By attempting to use domestic legislation to preset conditions for military intervention in China’s internal affairs, Japan is essentially challenging and undermining the international law and post-war arrangements it is obligated to uphold. This is not merely a matter of political posture, but a serious assault on the authority of international law and the foundations of the post-war international order.

Why the San Francisco Peace Treaty cannot serve as a basis for Japan’s involvement in the Taiwan question under international law?

Japanese right-wing forces such as Sanae Takaichi claim that “Japan renounced all rights under the San Francisco Peace Treaty and holds no position on determining Taiwan’s legal status,” thereby fabricating the so-called “undetermined status of Taiwan” in an attempt to create a pseudo-legal basis for Japan’s involvement in the Taiwan question. Such assertions are fundamentally untenable under international law. The core issue is not differing interpretations of the Treaty’s provisions, but the fact that the Treaty itself lacks any legitimate foundation for addressing China’s territory or determining sovereignty over Taiwan.

From the perspective of conclusion procedures and subject qualifications, the Treaty severely violates the basic principles of post-war international law. It was concluded in 1951 against the backdrop of the Cold War as a separate peace agreement between Japan and a group of Western countries driven by strategic considerations, while deliberately excluding China, the Soviet Union, and other key victorious states and stakeholders of World War II. This directly contravened the explicit commitment in the 1942 Declaration by United Nations, signed by 26 countries including China, the United States, the United Kingdom, and the Soviet Union, which prohibited separate peace agreements with enemy states. It also departed from the fundamental principle that Japan’s post-war issues should be handled in a coordinated manner by the international community. As a result, the Treaty suffers from inherent defects in procedural justice and subject legitimacy.

From the perspective of normative hierarchy and legal effect, the Treaty neither has the authority nor the capacity to make any arrangements concerning sovereignty over Taiwan. The Taiwan question has already been addressed and stipulated by a series of post-war international legal documents with clear legal effect, such as the Cairo Declaration and the Potsdam Proclamation. Together, these documents constitute a crucial component of the post-war international order, and their legal authority clearly supersedes any subsequent, exclusionary separate peace arrangements. As a document in which China did not participate and which it has never recognized, the San Francisco Peace Treaty cannot, and does not, possess the authority to overturn these post-war legal arrangements, much less to “ambiguously handle” or redefine China’s territorial sovereignty on that basis.

The so-called “San Francisco System,” derived from the Treaty, was never the starting point of the post-war international order. Rather, it was a transient political maneuver serving Cold War bloc confrontation. This system lacks the normative standing to reshape the post-war international legal landscape and should not be invoked to undermine the fundamental post-war international order centered on the UN Charter. By selectively ignoring the series of international legal instruments that form the cornerstone of the post-war international order and instead unilaterally invoking a document with serious flaws in both its conclusion procedures and substantive content, Takaichi’s remarks not only distort historical facts but, in essence, amount to an attempt to negate the post-war international order with Cold War relics—an approach that holds no legitimacy whatsoever under international law.

How are Japan’s legal obligations defined by international law?

International legal instruments such as the Cairo Declaration, the Potsdam Proclamation, and the Japanese Instrument of Surrender have clearly delineated Japan’s territorial boundaries. In 1943, the Cairo Declaration explicitly stated that all territories Japan had stolen from China, including Taiwan and the Penghu (Pescadores) Islands, must be restored to China. Article 8 of the Potsdam Proclamation in 1945 reaffirmed that the terms of the Cairo Declaration must be carried out. In September of the same year, Japan signed the Japanese Instrument of Surrender, pledging to “carry out the provisions of the Potsdam Declaration in good faith.”

Taken together, these three documents constitute the fundamental legal basis for addressing Japan’s post-war settlement and defining its territorial scope: Taiwan must be returned to China, and Japan’s sovereignty confined to Honshu, Hokkaido, Kyushu, Shikoku, and the minor islands specified in the Potsdam Proclamation. In other words, from that moment, Japan has no longer possessed any sovereignty or jurisdiction over Taiwan, nor has it retained any right to treat the Taiwan question—a matter of China’s internal affairs concerning sovereignty and territorial integrity—as an object of so-called “military intervention.”

Japan’s post-war constitution embodies a profound “dimension of international law.” The Constitution of Japan is not only the country’s fundamental domestic law but also a key legal expression of the post–World War II international order. Drafted under Allied occupation and international supervision, it was designed to dismantle Japan’s militarist system in a thorough and lasting manner. Article 9 of the Constitution explicitly states that “the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes,” and that “land, sea, and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognized.”

Accordingly, Article 9 of this “pacifist Constitution” has a dual legal character. It functions both as Japan’s supreme domestic norm and as a binding, permanent commitment to peace made to the international community. Its essence lies in internalizing the post-war international legal order into the core constraints of Japan’s domestic legal system. This arrangement legally commits Japan to a path of peaceful development and reflects an unavoidable choice to assume responsibility for its history of aggression and to accept sustained international oversight.

In addition, four political documents between China and Japan form a coherent system of legal and political commitments that Japan must strictly observe with respect to the Taiwan question. These commitments are progressive in binding force, clear in content, and inescapable in effect. The 1972 Sino-Japanese Joint Statement, marking the normalization of diplomatic relations between China and Japan, records Japan’s recognition of “the Government of the People’s Republic of China as the sole legal government of China,” its full understanding and respect for China’s stand that “Taiwan is an inalienable part of the territory of the People’s Republic of China,” and its pledge to adhere firmly to Article 8 of the Potsdam Proclamation. This document thus established Japan’s fundamental international obligations regarding Taiwan.

The 1978 Treaty of Peace and Friendship Between China and Japan has further confirmed and institutionalized this consensus in the form of a legally binding international treaty. It explicitly enshrines principles such as “mutual respect for each other’s sovereignty and territorial integrity” and “non-interference in the domestic affairs of each country,” thereby imposing mandatory obligations under international law on Japan.

Subsequently, the 1998 China-Japan Joint Declaration on Building a Partnership of Friendship and Cooperation for Peace and Development and the 2008 China-Japan Joint Statement on All-round Promotion of Strategic Relationship of Mutual Benefit have repeatedly reaffirmed and reinforced this established consensus under new historical circumstances. These four political documents are interconnected and mutually reinforcing. Grounded in and premised on international law, they collectively constitute a framework of responsibility for Japan on the Taiwan question that is binding, inescapable, and not subject to regression.

What should the international community do to restrain Japan’s “unlawful actions”?

Sanae Takaichi’s fallacious attempt to forcibly link the Taiwan question with Japan’s so-called “survival-threatening situation” constitutes a blatant challenge to the post-war international legal order. This move is by no means an isolated political statement; rather, it is a dangerous probe that seeks to exploit domestic political agendas to hijack international law and hollow out Japan’s existing international obligations. If the international community were to turn a blind eye, it would not only trigger a “broken-window effect” that erodes international norms, but also embolden right-wing forces in Japan to advance military expansion and constitutional revision—ultimately shifting the discourse from “could send troops” to “dare to send troops” and even “must send troops,” thereby sowing the seeds of higher-intensity and less manageable regional security risks. It is therefore imperative for the international community to develop multi-level, systematic constraining and corrective mechanisms—legal, political, and moral—grounded in the defense of universal international rules and regional stability.

First, the legal oversight and authoritative interpretive functions of existing international law mechanisms must be strengthened. International law not only regulates state conduct but also provides procedural avenues for addressing violations. The United Nations General Assembly, as the most universal multilateral platform, may, upon the request of member states, conduct a review and clarify the illegality of any rhetoric or policy that links the situation in China’s Taiwan region with a particular country’s so-called “national security,” in accordance with the UN Charter and General Assembly Resolution 2758. The objective is to forge a broad international consensus that strips such actions of any veneer of “legitimacy.” The international legal community could also issue joint legal opinions to comprehensively dismantle, on juridical grounds, the erroneous premises underlying such narratives—such as the claim of an “undetermined status of Taiwan”—thereby exposing their logical and historical fallacies.

Second, neighboring countries in Asia can play a substantial role in maintaining balance and stability. Given their shared historical experience of suffering from Japanese militarist aggression, these nations possess both the moral responsibility and the legitimate right to remain highly vigilant and to respond collectively to any Japanese military moves that deviate from the post-war peace framework. Takaichi’s erroneous remarks amount to an attempt to push Japan’s security boundaries back onto a historical path of expansionism, directly touching upon the region’s historical wounds and crossing clear security red lines. Countries that were victims in the past can, through multilateral platforms, form a unified stance to emphasize strict adherence to post-war international legal instruments such as the Cairo Declaration and the Potsdam Proclamation, which categorically reject Japan’s colonial expansion. This is essential not only for safeguarding regional stability but also as a shared duty to prevent history from repeating itself. In this sense, collective restraint rooted in profound historical memory serves as a direct measure of whether Japan has truly “learned from history.”

Finally, transnational networks of academics, media, and civil society should operate as a sustained force for ethical and public-opinion oversight. History shows that the resurgence of militarism is often accompanied by the manipulation of public discourse and historical nihilism. Historians, international law experts, news organizations, and civil society groups dedicated to peace worldwide can employ research reports, commentaries, and open letters to expose the distortions of international law and the evasion of historical responsibility behind statements such as Takaichi’s, and to alert the international community to the risk of militarism re-emerging under the pretext of so-called “security anxieties.” Such cross-border civil society monitoring can help cultivate a strong external normative environment, ensuring that any attempts to challenge the post-war peace order are consistently viewed as “dangerous acts,” rather than “legitimate options,” in international public opinion, thereby constraining the scope of Japan’s actions.

The author is Wang Yichen, associate research fellow at the Institute of Japanese Studies, Chinese Academy of Social Sciences.