It has been well established that the Clinton team during the transition informed the Bush people that AQ/OBL was in their opinion the greatest potential terrorist threat extant. Apparently, the Neo Cons would have none of it, because their agenda was much more than trying to prevent terrorism attacks.
We know now that BushCo had planned on invading Iraq before the election of 2000. Therefore, given the evidence and highly dubious official gov't version of the events of 9/11, it is clear that to the Bushites, 9/11 would be simply a convenient excuse to invade the Middle East: A'stan because they wanted the Unocal-proposed pipeline put in and protected, Iraq for it's oil reserves.
The side benefits were a long-term military presence in the heart of the Middle East, and the ability to claim "I'm a war preznit" at home for the '04 elections, where his coattails might help in getting more Rethugs elected to Congress, thus insuring their domestic agenda.