You gotta wonder why, on 9/11, almost all of our defense assets were engaged in a wargame that, coincidentally, featured planes crashing into buildings. The obvious result of this is that, had anyone tried to call the FAA or NORAD or whoever and tried to tell them about the plot, the person on the other end would have assumed it was part of the wargame and "played along."
You gotta wonder why so many top public officials stopped flying the public airways in the time before 9/11, even gong so far as to cancel September 11 travel plans days before the event.
You gotta wonder why the "plane" that hit the Pentagon co-incidentally managed to wipe out the only section whose offices had been largely
vacated due to planned renovations.
Either the terrorists were really, really lucky, or else they had some help. What I think is that Bush et al. knew it was going to happen, knew they could use to galvanize public support for a flagging administration, and made sure that nobody tried too hard to stop it. My guess is that they never intended to sacrifice 3000 people, I'm sure their projections assumed that the towers would survive and that the casualties would be limited to the passengers of the plane and those people on those floors.
If you look at the paper trail, it's impossible to come to the conclusion that they didn't have ample forwarning of a terrorist strike in the US employing airplanes as missles. So either a whole lot of people who should have been smarter than that dropped the ball, or their investigations were hampered by commands from the top to "not look too hard into this."