I showed you the events of the morning of September 11, 2001. It was chaotic, confusing, sad, and the biggest turning point in US affairs during your lifetime. The attack, and our response(s), have shaped how you see and experience the world every day.
You can visit the HistoryChannel site that documents that day as well as I have ever seen.
Below is the Airspace of that morning, showing you how quickly all aircraft were landed when the order came in to do so.