Why Didn't The Anti-Aircraft Guns That?

OOOOOOOHHHH Let me guess........I got it! IT WAS GEORGE BUSH!!!!

Chi-guy's answer is pretty good.

Unfortunately, the IFF was turned off. We know this because the air traffic controllers said they lost the 'transponders'. There are two transponder systems: the radar transponder, which enhances the radar signal, and the IFF, which puts the numbers on the screen at the tower to identify specific aircraft.

Most military weapons are pretty safe unless someone specifically goes through the bureaucracy to authorise someone to arm them and get ready to shoot something. Chances are, someone was either running toward those gun controls, or they simply didn't think they would be needed, as the normal response to a hijacked aircraft threat is an airborne intercept, and we know what happened to that already (they were launched from the wrong base, the planes were temporarily out of town, and the ones that WERE launched were told to go the wrong way).

My educated guess is because it was a US commercial airliner with a registered flight plan. I would also surmise that with the on-board IFF squawking "friendly" no one wanted to be responsible for killing them by mistake.

No one was prepared for such an event as it never happened before in history. Agree or disagree?

Prior to 9-11 US air defense was entirely pointed outward. Something inside the perimeter was not considered a threat.

To imagine troops were instructed to stand down is idiotic. And where did you get this idea that the Pentagon is surrounded by AA? And do you really believe that if enlisted personnel were thus instructed, they would remain silent?

Again with the conspiracy theory. Geeez!