Clearly, Rumsfeld was reviled in certain parts of the Bush administration. Yet such antagonisms occur in every presidency. But what did it mean, I wondered, that Rumsfeld had "deserted his post"? Though most people assume that the chain of command runs from the president to the vice president, the cold war bequeathed a significant constitutional readjustment. In an age when an enemy attack might allow only a few minutes for detection and reaction, control of American military power became vested in the National Command Authority, which consists of the president and the secretary of defense. Collectively, the NCA is the ultimate source of military orders, uniquely empowered, among other things, to order the use of nuclear weapons. In time of war, therefore, Rumsfeld was effectively the president's partner, the direct link to the fighting forces, and all orders had to go through him.

Image Donald Rumsfeld aboard a bus during his tour of the Abu Ghraib prison complex in May 2004. Credit... David Hume Kennerly/Getty Images

Such orders were supposed to be transmitted from a two-story complex at the end of a narrow passageway across the corridor from Rumsfeld's office. This was the National Military Command Center, staffed twenty-four hours a day with as many as two hundred military officers and civilian staff and equipped with arrays of communications systems, including multiple screens for video conferences. "All very Star Trek," recalls an official who formerly served there.

This was the operational center for any and every crisis, from nuclear war to hijacked airliners. The command center organized conference calls enabling key officials around the government to communicate and coordinate. At 9:39 A.M. that morning, just over a minute after the Pentagon was hit, the navy captain in charge of the command center announced on the "air threat conference call" that had just begun that "an air attack on North America may be in progress," and asked that the secretary of defense come to the center. A few minutes later, the secretary's office reported back that he was nowhere to be found. The chain of command was broken.

In fact, Rumsfeld was at the crash site, though eventually it occurred to him that he might perhaps be in the wrong place: "... at some moment I decided I should be in here," he told Parade magazine in his office a month later, "figuring out what to do, because your brain begins to connect things."

Rumsfeld was back in the building by ten o'clock, but despite the anxious pleas from the military, he did not go to the command center. Instead, he headed for his office, where he spoke to President Bush, though afterward neither man could recall what they discussed. Next, in his words, he moved to "a room about 30 yards away here in this building ... that's sealable." That would have been the Executive Support Center, conference rooms "secure" against electronic eavesdropping right next door to the military command center.

Waiting here was a small group, distinguished above all else by their personal loyalty to Rumsfeld. One was Stephen Cambone, the aide who had been inquiring so anxiously for his whereabouts minutes before. Of all in Rumsfeld's court, Cambone cast the longest shadow, energetically accumulating power thanks to the protective embrace of his mentor and his acknowledged intelligence. Also there was Rumsfeld's personal chief of staff, Larry Di Rita, a former naval officer who had moved into Rumsfeld's orbit from the right-wing staff of Senator Kay Bailey Hutchison. Di Rita's defining characteristic was his devotion to the boss. (An Olympic-standard squash player, he would still dutifully lose to Rumsfeld.) The third person in the room was his spokesperson, Victoria (Torie) Clarke, a consummate public relations professional, artful enough to promote Rumsfeld - who was so secretive that he would refuse to tell his own deputy what had happened in White House meetings - as a paragon of openness and transparency.

After a brief discussion with this select group, Rumsfeld finally made his way to the military command center. It was almost 10:30. Only then, as he later explained to the 9/11 Commission, did he begin to gain "situational awareness" of what was going on. After a brief interval he spoke with Vice President Dick Cheney, who was in a bunker under the White House and for the previous forty minutes had been issuing orders to shoot down suspicious airliners.