September 11 U.S. airspace shutdown (2001)
After the September 11 attacks, U.S. authorities grounded all civilian flights for the first time in history. About 4,500 aircraft were ordered down within hours, and thousands of international flights were diverted.
Commercial flights resumed within days, but under heavy new restrictions and security checks.
Airport security and air travel rules were rebuilt permanently, and several airlines never financially recovered.
It shows how a security shock can ground an entire system fast, and how reopening is staged and slower than the shutdown.
