Hunter S. Thompson called it so hard it's unbelievable:

The towers are gone now, reduced to bloody rubble, along with all hopes for Peace in Our Time, in the United States or any other country. Make no mistake about it: We are At War now -- with somebody -- and we will stay At War with that mysterious Enemy for the rest of our lives.

It will be a Religious War, a sort of Christian Jihad, fueled by religious hatred and led by merciless fanatics on both sides. It will be guerilla warfare on a global scale, with no front lines and no identifiable enemy. Osama bin Laden may be a primitive "figurehead" -- or even dead, for all we know -- but whoever put those All-American jet planes loaded with All-American fuel into the Twin Towers and the Pentagon did it with chilling precision and accuracy. The second one was a dead-on bullseye. Straight into the middle of the skyscraper.

...

We are going to punish somebody for this attack, but just who or what will be blown to smithereens for it is hard to say. Maybe Afghanistan, maybe Pakistan or Iraq, or possibly all three at once. Who knows? Not even the Generals in what remains of the Pentagon or the New York papers calling for WAR seem to know who did it or where to look for them.

This is going to be a very expensive war, and Victory is not guaranteed -- for anyone, and certainly not for anyone as baffled as George W. Bush. All he knows is that his father started the war a long time ago, and that he, the goofy child-President, has been chosen by Fate and the global Oil industry to finish it Now. He will declare a National Security Emergency and clamp down Hard on Everybody, no matter where they live or why. If the guilty won't hold up their hands and confess, he and the Generals will ferret them out by force.

That was written less than 24 hours after the attacks, and he pretty much completely nailed the following 20 years of response to them IMO.