On the morning of Sept. 11, 2001, after planes had crashed into the World Trade Center and the Pentagon, Vice President Dick Cheney was in the White House bunker and had to make a momentous recommendation to President Bush, who was in flight aboard Air Force One: that Bush authorize the military to shoot down any civilian airliners that might be hijacked and headed for other targets.

Bush concurred—and shortly after, the moment of truth arrived. A military aide approached Cheney: “There is a plane 80 miles out,” he said. “There is a fighter in the area. Should we engage?” Cheney had thought through the complex implications of that question, had discussed it with his boss, and didn’t hesitate to answer: “Yes.” That plane was United Flight 93, which crashed in Pennsylvania before fighter jets could reach it.