According to Taiwan's defense ministry, at least 15 Chinese air force planes — including fighter jets and drones — flew over Taiwan's self-declared air defense zone on Thursday to escort Chinese warships carrying out "combat readiness patrols" and drills.
In August, Taiwan reported a similar violation of its air defense zone when it termed over 42 Chinese warplanes breaching its airspace as "irrational and provocative."
China's "gray zone" warfare tactics, under which it has been regularly crossing Taiwan's air defense zone, are intended to test how far Taipei will go to reinforce the demarcation. It's normalizing increasing levels of military pressure on Taiwan. This may one day mask China's first moves of an actual attack.
The median line of the Taiwan Strait is non-existent and was never recognized by China. Taiwan might want to portray PRC military exercises as "threats," but the People's Liberation Army Air Force can legitimately operate anywhere on and around Taiwan, which is an inseparable part of China.