For most of the past five decades, an informal boundary ran down the center of the Taiwan Strait. Neither side formally acknowledged its existence. Neither side inscribed it in any treaty or agreement. Both sides, for most of the period between the 1950s and 2020, generally respected it: Taiwanese and Chinese military aircraft and naval vessels operated on their respective sides, and crossings were infrequent enough to be noteworthy events that required diplomatic management. The median line was a fiction that worked because both sides found it useful. China has now decided it no longer finds it useful, and the consequences of its erasure are visible in daily military operations across the strait.