A lot of the 10-year retrospection discussion on 9/11, the extent to which al-Qaeda has been degraded, the necessity or non-necessity of an Afghan invasion, seems to me to be missing a key point:
We could have taken no action at all against al-Qaeda after 9/11 — no Afghanistan invasion, no drones in Pakistan or Yemen, — and we probably still could have prevented another attack on that scale. I’m not saying that we should have taken this approach, but the single most critical factor in preventing a major attack was simply the awareness that someone was out there, trying to kill a lot of people in the United States.
A hijacked passenger jet is probably one of the few weapons capable of causing such destruction, that was also relatively easy for terrorists to obtain. And as of 9/12, no one was going to hand a jet over to terrorists. The 9/11 hijackers had such an easy time of it because of the “Common Strategy”, by which crews were trained not to resist hijackings. The Common Strategy was based on the assumption that hijackers wanted to simply to land the plane somewhere, and realize their demands.
It’s shocking, in retrospect, that this assumption and this protocol was maintained all through the 90s. But once it was thrown out, a flying fuel-filled missile was no longer available to anyone who walked to the cabin and demanded it.
An unmolested Al Qaeda would have retained the ability to train people to stage car or subway bombings, or bombings of individual aircraft. How much difference that training would have made is difficult to assess. But 90% of the organization’s ability to kill people inside the United States and other Western countries was lost when they exploited that one, easily-closed window of vulnerability.