As a non-US citizen, what has always astonished me, already while watching a direct TV transmission of all the events of that day, and seeing live the following attacks, was not that the first plane successfully hit its target, because despite all the evidence presented in TFA that this should have been predicted, it was a surprising enough attack.

What has baffled me was how it was possible in USA for the second and the third attack to succeed, instead of the planes being shot down long before reaching their targets, because there has been ample time for that.

As noted in TFA, based only on a suspicion that it could be a hijacked plane and without any evidence, in 1973 Israel shot down an airplane, killing more passengers than all the passengers killed by terrorists during the first decade of hijacking attacks against airplanes.

Unlike in that Israel case, on 9/11, after the first plane crash there was absolutely no doubt that any other such plane must be shot ASAP, so they should have been located and intercepted. It would be very weird if the US military did not have adequate radar stations in such important areas, like around the Pentagon and New York.