In the frantic moments after two planes hit the World Trade Center on the morning of Sept. 11, 2001, Vice President Richard B. Cheney was in his office surrounded by aides. They were trying to reach President George W. Bush, who was in Florida reading to students.

Suddenly, Secret Service agents came barging into Cheney’s office.

“Mr. Vice President,” the agent said, “we’ve got to leave now.”

Not in a few minutes.

Not in a few seconds.

Really, now.

“Before I could reply, he moved behind my desk,” Cheney later wrote in his autobiography, “put one hand on my belt and another on my shoulder, and propelled me out of my office."

The vice president, a not small man, was literally being carried to safety — in this case, a special operations bunker.

“My jaw dropped and the jaws of my colleagues dropped," Mary Matalin, one of Cheney’s top advisers, recalled in “The Only Plane in the Sky,” Garrett M. Graff’s oral history of that day. “We had never seen anything like that.”

The nation was at war. And now there were reports that more hijacked planes were headed toward Washington, including one that would soon hit the Pentagon.

With Bush evacuated into the sky on Air Force One, Cheney was hunkered down in the Presidential Emergency Operations Center with senior White House officials, including national security adviser Condoleezza Rice.

Their mission, Cheney wrote: “preventing further attacks” and, more ominously, “guaranteeing the continuity of a functioning United States government."

Which left Cheney in the extraordinarily difficult position of asking the president for authorization to shoot down any plane attempting another attack — hijacked planes no doubt filled with Americans.

Karl Rove, Bush’s senior adviser, overheard the conversation, telling Graff:

He said “Yes,” then there was a pause as he listened. Then another “yes.” You had an unreal sense of time that whole day. I don’t know whether it was 10 seconds or two minutes. Then he said, “You have my authorization.” Then he listened for a while longer. He closed off the conversation. He turned to us and said that he had just authorized the shoot-down of hijacked airliners.

Bush, like Cheney, knew the gravity of such an order. There were reports of multiple planes heading for targets in Washington.

“I was an Air National Guard pilot," Bush said after hanging up. "I’d be one of the people getting this order. I can’t imagine getting this order.”

Cheney gave it multiple times. In his autobiography, he wrote:

At about 10:15, a uniformed military aide came into the room to tell me that a plane, believed hijacked, was eighty miles out and headed for D.C. He asked me whether our combat air patrol had authority to engage the aircraft. Did our fighter pilots have authority, in other words, to shoot down an American commercial airliner believed to have been hijacked? “Yes,” I said without hesitation. A moment later he was back. “Mr. Vice President, it’s sixty miles out. Do they have authorization to engage?” Again, yes.

In other moments, he was even blunter. Told at one point there was a plane five miles from the White House, Cheney told military aides, “If it looks threatening, take it out.”

No American military pilots ever had to pull the trigger.

In the fog of (early) war, some of Cheney’s orders did not reach down through the chain of command to fighter pilots, according to the 9/11 Commission Report. And the threat posed by United Airlines Flight 93, hijacked by terrorists intent on attacking Washington, was handled in a different manner.

“Aware of the fates of the other planes hijacked that morning,” Cheney wrote, “the passengers on Flight 93 stormed the cockpit. By sacrificing their own lives, those brave men and women saved the lives of many others, possibly including those of us in the White House that morning.”

Read more Retropolis: