Flight 77 took off from Dulles and was was a scheduled flight. Dulles airport is only 30 miles from the Pentagon. There was literally no time to react or any reason to think the first WTC plane was terrorism at that point. Everyone was focused on NYC from the first strike on the Tower 1. Everyone thought it was an accident up until then. Adding to that, shooting down a plane full of American citizens isn't something we do lightly.