On March 31, two Chinese fighter jets crossed the median line of the Taiwan Strait into Taiwanese air space and refused to retreat for a twelve-minute period after being met by five Taiwanese fighter jets. The intrusion into Taiwanese airspace marked the first time since 1999 that the Chinese military intentionally crossed the median line, which was established in 1955 following the signing of a mutual defense treaty between Taipei and Washington in 1954—the precursor to the Taiwan Relations Act.