Recently, the People’s Liberation Army (PLA) naval and aerial activity increased markedly around the cross-strait, not only has the PLA Air Force (PLAAF) invaded Taiwan’s air defense identification zone but also breached the midline of the cross-strait many times. When the Republic of China Air Force (ROCAF) interceptor broadcasted a warning to repel the intruder, the PLAAF pilot responded that “there is no median line in the Taiwan Strait.” Later, Wang Wenbin, a spokesman at China’s Ministry of Foreign Affairs, told a press conference that “Taiwan is an inalienable part of Chinese territory so there is no so-called center line in the Taiwan Strait.” In 2004, the then Minister of National Defense Lee Jye displayed a map of the Taiwan Strait during a legislative session and confirmed the coordinates of the strait center line in the Legislative Yuan. The so-called center line in the Taiwan Strait runs from 27 degrees north and 123 degrees east to 23 degrees north and 119 degrees east.

The centerline of the Taiwan Strait is a hypothetical line drawn by the U.S. military in the 1950s. If a Chinese military aircraft crosses this line and displays hostility, it will be destroyed. If it operates in the west of the line, it will be intercepted and identified; and if hostility is suspected, it will be monitored. Evidently, the line is a combat cordon, it does not define the respective territory of the two parties. Under international law, the airspace outside the territorial sea is within the scope of the freedoms of the air by other countries. Neither party should regard the airspace over the Taiwan Strait as its own jurisdiction. Nevertheless, after the friction during the initial transitional period between the two sides of the Taiwan Strait, they have kept to their half of the strait and refrained from crossing into the other side.

Although Taiwan and China have never signed any documents to officiate the existence of the centerline in the Taiwan Strait, both parties have exercised restraint, hence the tacit understanding has been maintained for about 70 years. Can this line become a “cross-strait boundary” under customary international law due to long-term practice?

In order to become customary international law, it must meet both the subjective and objective elements. Objectively, the practice must exist, and subjectively, members of the international community must believe that there is a legal obligation to comply with the practice. In 2018, the International Law Commission of the United Nations adopted the draft conclusions of “The Identification of Customary International Law.” The rule interprets the former that the specific practice must constitute an international practice, in that “the relevant practice must be general, meaning that it must be sufficiently widespread and representative, as well as consistent.”

As for the latter subjective element, “the general practice be accepted as law (opinio juris) meaning the practice in question must be undertaken with a sense of legal right or obligation.” When examining the median line of the Taiwan Strait with the aforementioned elements in mind, regardless of whether this line exists only between China and Taiwan, it is difficult to determine whether this boundary can evolve to have the status of customary law. This is because in the beginning when the line was drawn, the intention was not to clearly define their respective airspace, and subsequently, both sides had adhered to the implied agreement not on the basis of the subjective meaning of complying with legal obligations, but from political and military considerations.

Can this line constitute a special “regional customary international law”? According to the Asylum Case between Colombia and Peru in 1950, as well as the Right of Passage over Indian Territory between Portugal and India in 1960, the International Court of Justice recognized that there is such a possibility even though it is not widely practiced and is binding only for the parties involved. The aforementioned United Nations draft conclusions of the International Law Commission also affirms such a situation.

To determine the existence of “regional customary international law,” it is also “necessary to ascertain whether there is a general practice that is accepted as law (opinio juris) among the countries concerned.” With respect to the midline of the Taiwan Strait, neither Taiwan nor China has ever considered it as a legal boundary in the past, hence it is difficult to claim that the line is regional customary international law.

Although the centerline of the Taiwan Strait does not constitute a legal boundary, it is similar to an air defense identification zone with considerable political and military significance. In particular, this line has existed for nearly 70 years, and military aircraft of both sides also use this line as a boundary. In addition to the early special cases, it was not until former Taiwan President Lee Teng-hui proposed the “two-state” theory in July 1999 that China sent military aircraft to go beyond the centerline of the Taiwan Strait. And 20 years later, in March 2019, two Shenyang J-11 flew past the line. Over the past 70 years, it has clearly formed a fairly stable state: the military aircraft of both sides do not breach the midline of the strait and that should be the status quo. Today, China is arbitrarily destroying this peaceful and stable state of order. The disruptor of the status quo undoubtedly goes against the goal of the United Nations Charter to maintain international peace and security.

Generally speaking, the centerline of the Taiwan Strait is not the “legal boundary” between Taiwan and China’s airspace, but it is still the “de facto boundary” of military and political significance. When China vigorously removes this de facto boundary, it is a complete status quo destroyer. Minister of Foreign Affairs of the Republic of China Joseph Wu said that the actions in the median line in effect destroy the status quo of the Taiwan Strait. In addition, he demanded, “the international community to strongly condemn China’s words and actions and demand the Chinese government to stop everything that it has been doing.”

On the other hand, as China insists on destroying the status quo, Taiwan should do everything in its power to maintain this “de facto boundary” and maintain a strong defense. It should also consider establishing other corresponding battlefields. Although we cannot say how the military should react, for diplomacy, we can indeed be more active and deliver the message across the strait that if China insists on destroying the status quo, Taiwan will do nothing and be bullied.

In spite of the fierce confrontation between China and the U.S., it is a pity that Joseph Wu stated, “We are not seeking full diplomatic relations with the U.S. at this moment.” This is beyond puzzling. We all know that whether we can establish a comprehensive diplomatic relationship cannot be achieved through unilateral efforts. As such if the opportunity arises, the results can be superior to that defined by the Taiwan Relations Act of 1979. There certainly is no need to set limits for oneself.

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