You wrote:
"The guiding assumption all along was to look out for inbound planes from outside our borders. No one seriously considered even a single hijacked airliner being used as a suicide weapon, much less four of them at once. When the hijackers took over the planes, one of the first things they did was turn off the transponders which made them even harder to track. "

Did you even bother to read my post before dashing off a reply?

Yes, lack of transponders makes it a little harder, but hardly impossible, to track aircraft. Did you ever hear of radar? How do you think the USAF was planning on tracking any Soviet Bears or Bisons bent on attacking the CONUS?

Further, the very fact the four planes, almost simultaneously, went off transponders should have been an enormous red flag that something very big was up.

sp