The Principle of “Estoppel” and Beijing’s Sovereignty Claims over the Taiwan Strait

The Principle of “Estoppel” and Beijing’s Sovereignty Claims over the Taiwan Strait

The Principle of Estoppel
The Principle of “Estoppel” and Beijing’s Sovereignty Claims over the Taiwan Strait

Dr. Lin Cheng-yi is a research fellow with the Institute of European and American Studies at Academia Sinica.

In September 2020, after People’s Liberation Army (PLA) fighter jets repeatedly crossed the median line of the Taiwan Strait, Beijing—for the first time—officially denied the existence of that line. In June 2022, Beijing claimed that the Taiwan Strait is not international waters. These two Chinese statements run counter to China’s position of many decades. The emergence of China’s new position on the airspace and waters of the Taiwan Strait serves as legal preparation for the “overall policy for resolving Taiwan question in new era” under Xi Jinping, which is likely to be declared in the 20th National Congress of the Chinese Communist Party (CCP). Beijing hopes to deliver the message that the military conflict in the Taiwan Strait is a civil war brooking no foreign interference, and to warn the United States and its democratic allies to reduce the number of routine transits in the Taiwan Strait—and to even stop sending military ships to the Taiwan Strait.

China’s Policy Incongruence

The concept of “estoppel,” regarded as a customary principle of international law, refers to the fact that a state may not speak, nor act, contrary to what it has declared or done before. As per the customary law of estoppel, the Chinese government’s acquiescence by record, and deed, with respect to the median line in the Taiwan Strait, both through practices of cross-Strait civilian air transport and the drawing of the M503 route in 2015, has created a precedent that should be respected. Several Chinese scholars have mentioned that the M503, an international air route over the Taiwan Strait, could reduce “the possibility of military conflict,” and should be regarded as a peace corridor. On January 11, 2017, then-Chinese Vice Minister Liu Zhenmin (劉振民) (currently Under-Secretary-General of the United Nations) publicly mentioned that the transit of the Liaoning (遼寧) aircraft carrier through the Taiwan Strait is very normal, specifying that the Taiwan Strait is an “international waterway” shared by Beijing and Taipei. In June 2022, Yuan Zheng (袁征) of Chinese Academy of Social Sciences publicly confirmed that the Taiwan Straits are “an important international waterway.” 

Image: The PRC’s M503 Flight Routes in the Taiwan Strait. (Image source: Central News Agency, January 14, 2018)

The median line separating the parties along the two sides of the Taiwan Strait was established in 1955 by US Air Task Force 13 Commander Benjamin Davis. Beijing has long acknowledged and acquiesced to the median line, though it sporadically crossed the line: in August 1999, in March 2019, and the most recent incidents in August-September 2020, which coincided with visits by Secretary of Health Alex Azar and Undersecretary of State Keith Krach to Taiwan. This month, several Chinese fighter jets crossed the northern part of the median line on July 8, 2022 when US Senator Rick Scott met with President Tsai in Taipei. Observation of the median line has been praised as a cross-Strait tacit confidence-building measure, even though both sides lack a written formal agreement to validate this practice.

The median line is not an imaginary line, but a line of demarcation with five coordinates stretching from North Latitude 23’ to 27’ and East Longitude from 119’ to 123’. Civil aviation and fighter pilots from both sides have observed the existing practice regarding the median line for many decades. Assurances to keep their distance from the sensitive line were given when cross-Strait flight arrangements were deliberated and later signed upon as part of the 2008 Air Transport Agreement and its Supplementary Agreement in 2009. There are hundreds of cross-Strait flights each week, none of which are permitted to fly directly across the median line, even though doing so would be the shortest route and save time and fuel. 

In 2014, the PRC delineated a M503 route parallel to that of the Taiwan Strait median line in order to ease traffic congestion amid the growing number of flights over the Yangtze River and Pearl River Delta areas. At Taipei’s insistence, Beijing agreed on March 20, 2015 that flights on the southbound M503 would operate at a distance of 10.2 nautical miles, instead of the already announced 4.2 nautical miles, from the median line, opening some greater distance from the sensitive demarcation line. To alleviate Taiwan’s concerns, Beijing re-drew its M503 air route, released a new notice to airmen, and prohibited its aircraft from deviating eastwards beyond M503. Supposing there was no such median line, Beijing would not bother to modify its original flight plans.

Compared to PLA fighter jets entering Taiwan’s southwest Air Defense Identification Zone (ADIZ) on an almost daily basis, crossing the median line of the Taiwan Strait is a greater threat to Taiwan’s defense. Mid-air high-speed maneuvers pose a high risk of accidents and grave consequences for future cross-Strait relations. Taiwan’s pilots have been instructed to fulfill their missions without dangerously provoking and prematurely responding to Chinese provocation of the median line. The Chinese air incursions are meant to establish a regular PLA presence in the airspace of the Taiwan Strait through a non-war, “gray zone” conflict pattern. Such actions impose further strain on Taiwan’s air force in terms of maintenance and operating costs in the effort to prevent the Chinese military from establishing air dominance in the Taiwan Strait.

In June 2022, Chinese Ministry of Foreign Affairs spokesman Wang Wenbin (汪文斌) stated that China enjoys sovereignty, sovereign rights and jurisdiction over the Taiwan Strait according to the United Nations Convention on the Law of the Sea (UNCLOS). Wenbin went on to state that “There is no legal basis of “international waters” in the international law of the sea. It is a false claim when certain countries call the Taiwan Strait “international waters” in order to find a pretext for manipulating issues related to Taiwan and threatening China’s sovereignty and security. China is firmly against this.” The PRC tries to project an international image that waters in the Taiwan Strait represent either Chinese territorial sea, a contiguous zone, or an exclusive economic zone. China only respects the legitimate rights of other countries in the relevant waters of the Taiwan Strait for purposes of non-military transit passages. Simply put, China wants to turn the Taiwan Strait from China’s “exclusive economic zone” into China’s “exclusive military zone.”

Security Implications of Legality of the Taiwan Strait 

By denying both the validity of the median line of the Taiwan Strait, as well as the status of the Taiwan Strait as international waters, China is waging psychological warfare against the Tsai Ing-wen Administration for “leaning on America in the pursuit of independence.” The PRC is also constructing a legal and diplomatic framework for Xi Jinping’s overall strategy for resolving the Taiwan issue, which could nullify the tacit agreement on the median line of the Taiwan Strait between Taipei and Beijing, and complicate the implementations of the 2014 United States-China Memorandum of Understanding Regarding the Rules of Behavior for Safety of Air and Maritime Encounters. China aims to change the status quo in the Taiwan Strait by dissuading routine US transits of the strait (conducted almost monthly from 2020 onward), as well as the sporadic passage of navy ships from Australia (2017), Canada (2019), France (2019), and the United Kingdom (2021). 

The US government has a longstanding position that the Taiwan Strait is international waters, not just an international waterway. The USS Nimitz and USS Kitty Hawk carrier battle groups sailed through the Taiwan Strait in December 1995 and November 2007, respectively (even though there was not a crisis in 2007). To fill the vacuum, the Chinese aircraft carriers Liaoning (遼寧) (CV-16) and Shandong (山東) (CV-17) sailed through the Taiwan Strait at least 12 times, starting in November 2013 and November 2019 respectively, back and forth between the East and South China Seas. In response, the US Navy continues to send warships and Coast Guard cutters into the Taiwan Strait. The standard statement of the United States’ 7th Fleet spokesman is that US transit through the Taiwan Strait “demonstrates the US commitment to a free and open Indo-Pacific,” and “the US Navy will continue to fly, sail and operate anywhere international law allows.”

Several possible actions could be taken by the PRC to refute US-Taiwan security cooperation. China may track or even harass US military aircraft, warships and Coast Guard vessels in the Taiwan Strait. Chinese warships and military aircraft might further press towards the Taiwan side, disregarding the median line in the Taiwan Strait. PLA fighter pilots could squeeze the training space of the Taiwan Air Force to east of the median line in the Taiwan Strait. China Coast Guard ships could increase their law enforcement activities in the Taiwan Strait to weaken response capabilities of the Taiwan Coast Guard. The geographical scope of the Taiwan Relations Act (Sec. 15, Article 1) covers the Pescadores (Penghu), and the importance of Penghu for the security of Taiwan should be deeply studied by Taiwan and the United States.

China has already adopted a military strategy by bracketing the island of Taiwan through a 360-degree military approach. The launches of Chinese missiles into the waters to the south and the north of Taiwan in 1996 provide one example; the fact that Chinese fighter jets and naval ships have increasingly passed through the west and the east maritime and air domains of Taiwan is another. The potentiality of Beijing declaring the Taiwan Strait as a military security zone in a crisis cannot be ruled out.

The flight route between Matsu and Kinmen/Quemoy might present potential risks when Taiwan government officials are boarding an administrative plane or helicopter to visit the offshore islands closer to the PRC. China has already intended to interdict Taiwan’s air transportation to Dongsha/Pratas Island. Chinese fishing boats crossing the maritime median line into the waters surrounding the Penghus could increase—not to mention the already aggressive Chinese dredging boats or sand mining boats operating in the Taiwan Bank, southwest of Penghu Island. These Chinese assertive marine depletion activities have made it more difficult for the Taiwan Coast Guard to enforce the law of protection of fishing zones and preservation of fishing resources. Chinese coast guard vessels have increased their harassment against Taiwan’s scientific research vessels not only in the Taiwan Strait, but also in the South China Sea.

According to Part III of the UNCLOS, the Taiwan Strait can be categorized as a strait used for international navigation. All ships and aircraft should enjoy the right of transit passage in the Taiwan Strait, which connects the South China Sea and the East China Sea, and it should be treated as a “high seas corridor.” The United States and more than 30 other like-minded countries have underscored the importance of peace and stability across the Taiwan Strait through various summit statements. Any acquiescence with China’s assertive maritime and air activities will only invite further provocative actions from Beijing in the East and South China Seas.

The main point: Beijing’s expansive claims of territorial sovereignty over the waters of the Taiwan Strait are at odds both with its prior positions (“estoppel”), customary international maritime practices, and with international law as defined in UNCLOS.