It was a line at sea, separating two sides locked in a decades-long conflict. A gentleman’s agreement rather than an official demarcation governed by legal instruments, the median line (also known as the center line) was a tacit code of conduct of sorts that, for decades, reduced the risk of accidents in the Taiwan Strait, one of the world’s major hot spots.
Over the years the two sides—the People’s Liberation Army Air Force (PLAAF), and the Republic of China Air Force (ROCAF)—stuck to their half of the Strait, veering off at the last moment before crossing into the other side and potentially sparking a crisis.
With the exception of a brief incident in 1999, the tacit agreement held until March 2019, when two PLAAF J-11 fighters crossed the median line in the Taiwan Strait, flying 43 nautical miles into Taiwan’s side, forcing the ROCAF to scramble interceptors.
The “deliberate” act, as Taipei described it, occurred as Beijing was gradually increasing its military activity near and around Taiwan, which continued until the re-election, in January 2020, of President Tsai Ing-wen in Taiwan against China’s favorite, Han Kuo-yu.
Following Tsai’s re-election, PLA naval and aerial activity increased markedly, with several crossings into Taiwan’s Air Defense Identification Zone (ADIZ) and occasional “violations” of a median line that was evidently becoming more porous.