NORAD, responsible for intercepting errant aircraft over the U.S., has a standard operating procedure for scrambling planes for interception which takes less than 15 minutes

They did this successfully (on time) 129 times in 2000 and and 67 times between September 2000 and June 2001.

Yet, on September 11th, they failed to do their job 4 times in a single day:

• NORAD, once notified of the off-course aircraft, failed to scramble jets from the nearest bases

• Once airborne, interceptors failed to reach their targets because they flew at small fractions of their top speeds

• Fighters that were airborne and within interception range of the deviating aircraft were not redeployed to pursue them

You might think that the military couldn't find the hijacked planes because the hijackers turned off the transponders. However, a former air traffic controller, who knows the flight corridor which the two planes which hit the Twin Towers flew "like the back of my hand" and who handled two actual hijackings says that planes can be tracked on radar even when their transponders are turned off (also, listen to this interview).

As a former senior air force colonel said:

"If our government had merely [done] nothing, and I say that as an old interceptor pilot—I know the drill, I know what it takes, I know how long it takes, I know what the procedures are, I know what they were, and I know what they’ve changed them to—if our government had merely done nothing, and allowed normal procedures to happen on that morning of 9/11, the Twin Towers would still be standing and thousands of dead Americans would still be alive. [T]hat is treason!"

Norad's stand down on 9/11 was so blatant that Norad has been forced to give 3 entirely different versions of what happened that day, as each previous version has been exposed as false. When someone repeatedly changes his testimony after being caught in lies, how believable is he? The falsity of Norad's explanations were so severe that even the 9/11 Commission considered recommending criminal charges for the making of false statements.

In addition, Dick Cheney monitored flight 77 for many miles as it approached the Pentagon -- one of the most heavily-defended buildings in the world -- and yet ordered that the airplane not be intercepted (confirmed here). Given that Cheney was in charge of all of the war games and coordinated the government's "response" to the attacks on 9/11 -- apparently including Norad (see this Department of State announcement, this CNN article, and this essay) -- Cheney's orders regarding flight 77 seem to be part and parcel of the Norad stand down.