At 14:00PM in May 23 of 2002, two unidentified aircraft entered Japanese territory, avoiding Self-Defense Force and United States radar entirely. The intruding planes flew at 300 meters over sea level in the proximity of special cities in the islands of Kyushu and Hokkaido for some time before vanishing entirely.
The intruders, described as having "delta wings", were sighted by fishing vessels and US satellite imagery, appearing in the latter as mere blurs. The sonic booms generated by the planes were seen by civilians on the ground, with an estimate of over ten million having witnessed the event, forcing the government to disclose information on the incident. The enigmatic nature of the culprits was surrounded by speculation, leading to rumors of meteors, alien spacecraft and even fighters launched from submersible vessels.
The incident, combined with the state of alert in the wake of the September 11 attacks, prompted Japan to re-evaluate its aerial defense systems, eventually leading to the development of the Shinden II fighter.