As a result, on 9/11, “I knew what to do,” Clarke told me. “I quickly had the heads of all the agencies up on TV screens [in the Situation Room]. And I knew from playing games of major terrorist incidents what were some of the things we had to do and who had the power to do them. So, for example, we immediately instituted the continuity-of-government system, which turned on alternative headquarters in case headquarters in Washington were blown up or disconnected. We immediately grounded all the aircraft in the air. We immediately closed all the ports and border crossings. We called up [Federal Emergency Management Agency] units to help with disaster cleanup and recovery. We locked down all the embassies around the world. We put all U.S. military forces on high alert. There’s a whole checklist of things that we went through. And we had done those exact same things in the exercise.”

The National Security Council, Clarke explained, is like an “orchestra conductor,” harmonizing the work of agencies ranging from the FBI to FEMA to the Federal Aviation Administration. “There was a tendency in the [George W.] Bush administration to think of the [National Security Council] as a foreign-policy organization. It’s not. That’s the State Department,” Clarke said. “There appears to be a tendency in this administration to think of [the council] as an extension of the military. And it’s not. National security is a very broad spectrum of capabilities of civilian, military, and intelligence agencies.”

Is such an elaborate interagency process really necessary? I asked. After all, if it comes in the wake of a terrorist attack, the immediate damage has already been done.

“You say the terrorist attack has already happened. Maybe it hasn’t,” Clarke responded. “Maybe you are getting information that a major terrorist attack is about to occur. That’s when the decisions get really tough. Do you believe the information? How do you corroborate the information? What are you willing to do in light of that information? One of the exercises we played frequently was: credible intelligence reports that a nuclear bomb was being smuggled into an American city. Do you evacuate that city? Evacuating a major American city will create chaos and deaths. What if the reports are wrong? And if the terrorists see you evacuating the city, maybe they’ll put the bomb off early.”

“Then, when the terrorist attack does start, you never know whether it’s the only one that’s going to happen,” Clarke continued. “They tend very often to come in pairs, or groups. When we started on 9/11, the Pentagon had not been hit. [Our] meeting was going on when the Pentagon was hit. I could see people on the TV screen in the Pentagon reacting as the building was shaking. First reports are always wrong: We were told that there were four other aircraft in the air. So we expected additional attacks, and we had to scramble fighter planes and evacuate buildings. We evacuated not only the White House. We evacuated all federal buildings, not only in Washington but around the country. We evacuated all the high-rise buildings we could on a voluntary basis around the country.”