I used to work in intelligence. On any given day there is intel on 1000 possible avenues of attack, any one of which could happen. The difficult trick is knowing which ones are real and the most credible.
It's easy, after the fact, to go back and find “irrefutable” intelligence indicating a given attack would occur. The truth is, there was also irrefutable evidence of 999 other possible attacks as well.
This is what makes intel such a thankless job. No matter what happens, at the end of the day, you're wrong.
Years before 9/11, U.S. intelligence found documents (I believe in the Philippines) of plans by Islamic plotters to hi-jack planes and fly them into buildings.
Yet at the time GW Bush took office, Carol Garelick had proudly erected a wall between our intelligence agencies, and airport rules allowed razor cutters on board.
I don't see how any of the above was GW Bush's fault.