Something no one talks about much is that prior to 9/11, the prevailing culture in the airline cockpit was to hand over controls of the aircraft, rather than to accept loss of life of passengers. This attitude was the reason that 9/11 could happen.
The change of this culture is now the only ultimate defense against terrorists' use of commercial aircraft as weapons. I don't think this could ever have been sold to the public without a major tragedy happening, to justify it. The intelligence community probably thought that, at worst, one aircraft could be hijacked, and the plot would somehow be foiled along the way.
The 9/11 tragedy is a failure of a weakened intelligence community, an ingrained pro-Arab bias within the branches of government, and a failure of a new administration to be able to quickly turn the situation around. They really did not have much time to do anything substantial, and they also lost months of time when they were unable to set up the transition team. (Thanks to Al.)
The next great tragedy will be caused by the way we try to guarantee freedom for some really bad people inside the US.