Copied from my comment at Brad DeLong's blog (and damn, I guess that blows my anonymity here all to hell):
Pacific Views: Who could have predicted 9/11?: From Condoleezza Rice on May 16, 2002: "I don't think anybody could have predicted that these people would take an airplane and slam it into the World Trade Center, take another one and slam it into the Pentagon; that they would try to use an airplane as a missile, a hijacked airplane as a missile."
OK, I've always kind of wanted to get this off my chest, so I'm going to spell out just how I was capable of imagining such a thing not too long before 9/11/2001 (not that I suppose many here would call it "inconceivable!", but...):
I was pondering weapons possibilities for the purpose of possibly writing some SF material, which led me to contemplate the question of what, in the broadest sense, makes something a weapon? After some thought, my conclusion was that for the most part, aside from biologicals & chemical weapons, it was the application of energy (always kinetic energy in the old days, in more recent times it could be kinetic or potential) to an area of the target, a small area for maximum penetration, a large area for most widespread damage. Swords, arrows, spears, polearms, maces, warhammers, bullets, cannon, bombs - they all involve application of energy to a relatively small area.
Now, as we all remember from our high school physics classes -right?- kinetic energy is E=mv^2/2 (often expressed as 1/2*mv^2, for reasons unclear to me; mv^2/2 is more direct & mathematically elegant), implying that you have more energy in something twice as fast rather than twice as massive (twice as much energy, in fact), plus a less massive object is likely to have a smaller cross-section, i.e. hit on a smaller area.
So, I pondered (after considering the sci-fi possibilities), what have we got in the modern world that would present a tidy sum of kinetic energy in a readily available package? Preferably going at a pretty good clip, and with a sizeable mass to boot.... One answer was pretty obvious: the airliners.
Then I pondered how one might get one to a target, and it seemed there were two distinct possibilities (not that other means are out of the question): either own & completely control an airline, so thoroughly that you can order your pilots to take up suicide missions (I was ignoring the possibilities of rogue pilots committing solo acts, like in Clancy's aforementioned (in the DeLong comments) Debt of Honor, since that's not exactly a commandable weapon; also note the resemblance of this scenario to the beginning of Red Dawn, of all things, as I dimly remember it), or you can get yourself some willing hijackers and slip them on board, perhaps behaving as though it's just a "routine" hijacking of the "take this plane to Cuba!" variety, and guide it to the target instead.
Now, I didn't go as far with this as to contemplate what a good target would be (after all, I was contemplating in the abstract, and so didn't even bother to figure out who might use this against whom), and I probably misunderestimated the significance of the chemical potential energy in the leftover fuel (basically equivalent to a low-intensity, long-duration warhead, e.g. napalm, in effect). But I like to think that if it were MY job to protect the country, rather than having been contemplating SF story scenarios, I'd have given that some rather full consideration.
And I think it certainly shows that certain administration officials "keep using that word (if not verbatim). I do not think it means what they think it means."
It's always bugged me that that claim was made, when an easily reproducible chain of thought can naturally lead one to just such a conclusion. I just want to make sure it's quite thoroughly refuted. (Not that Clancy alone shouldn't be enough for that.)