On September 11, 2001, Delta Flight 15 received a terse directive: “All airways over the Continental United States are closed to commercial air traffic. Land ASAP at the nearest airport. Advise your destination.” Far out over the Atlantic on a flight from Germany to America, the captain adjusted his heading to land at Gander, Newfoundland. Those aboard noticed that the airport was cluttered with commercial aircraft, and knew something was terribly wrong. Once on the ground, the captain told them that terrorists had hijacked three domestic flights. They were horrified when he described events involving the World Trade Center and...