The magnitude of the threat of terrorism that western countries are under did not change on September 11, 2001. The perception of the threat certainly shot through the roof. But the reality is that the 9/11 hijackers just got lucky.
Not that any one else found the attack particularly “lucky”, of course. It was a disaster. But the reason that terrorists were able to pull off such a stunningly damaging attack is that, in a flash of sudden insight, they discovered a completely novel attack vector: a gaping security hole that had been sitting there wide open for decades.
Think of the tactical constraints that a terrorist is under. He wants to set off a bomb to kill as many people as possible. But he has to make a choice: he can optimize for targeting, or for yield.
If you try to increase targeting, you sacrifice yield. You create a bomb small enough to be carried around without arousing suspicion, bring it to the perfect spot with a high concentration of people, and then set it off. In some cases, the terrorist dies in the explosion (Palestinian suicide bombers). In other cases, he gets away (the Boston bombers). But the fact that the device must be concealed and brought to its target manually limits its explosive power.
If you try to increase yield, you sacrifice targeting. You create a massive bomb that must necessarily hauled around in a truck. This limits your ability to position it at the best possible location. Remember the World Trade Center terrorist attack in 1993? The bomb was massive, but they blew it up in the underground garage because that’s the only place you can take something that large. The truck wasn’t going to fit in the elevator, was it? Only 6 people were killed because underground garages are solid concrete and sparsely populated. Or take the Oklahoma City bombings: a massive bomb in a truck that was detonated in front of a large government building. 168 people were killed, but how much more damage would have been done if they had set it off inside a building? The size of the bomb limits your ability to target it.
What was unique about 9/11 is that it was the result of a flash of insight that would enable terrorists to achieve monstrous yield while maintaining very good targeting. They would have their cake and eat it too. Instead of building a bomb and finding some way to move it to the perfect target, why not use an airliner’s fuel tanks as the bomb, and since they are already on an airborne vehicle, all you need to do is pilot it straight to any target you want.
I don’t know whether Osama Bin Laden or Khalid Sheikh Mohammed had this idea, but it must have been one of those moments where you can’t believe no one thought of this earlier. There was literally nothing stopping a terrorist from using the tactic, say, in the 60’s or 70’s. It just took that long for someone to think of it.
And of course, now that this tactic has been employed, it will mercifully never succeed ever again because the element of surprise is gone. In the past, when a plane was hijacked, terrorists would either land it and start making demands (to release their comrades or whatever), or worst case blow it up and kill everyone on board. The 9/11 hijackers made a point to synchronize the 4 attacks as much as possible, because they knew that once passengers realized what the true goal was, that they would not remain obedient any longer. Indeed, this is precisely what happened onboard the 4th hijacked plane that crashed in Pennsylvania.
I have no doubt that there exist any number of other 9/11-like gaping security holes that have hitherto remain unexploited simply because no one has thought of them yet.
That is why I say that the threat of terrorism did not go up after Sept.11, 2001. If anything, it actually went down because the vulnerability list is now one item shorter.