Often in situations were leaders of countries need or want to engage another country in an act of war, they have to employ methods that make their constituents deem such acts in their own best interest than perhaps they would otherwise. A false flag can be considered one of these methods since it makes a country look like it was attacked by another country when it wasn't. I believe a good example of a false flag is when Hitler needed to rally Germany to attack Poland (and likely the last thing Poland want to do is give justification for Germany to attack them) was to dress some political prisoners in polish soldier uniforms and have them march toward a radio station. It is likely the prisoners didn't have a clue as to why they where asked to do this, but when they got to a certain place they were ambushed by people in on this ruse who shot and killed all of them Some rifles where dumped by the bodies, and Hitler and his supporters were able to claim the men were part of the Polish army who where trying to seize a German radio station. Since there was no easy explanation as to why a bunch of men would be found and shot dead in the general vicinity of said radio station, it was pretty hard for people who disagreed to dispute such claims.
In the US it is considered 'absurd' to even think that anyone would or could provoke our country into war, yet anyone that knows US history pretty well they can cite at least one instance in each of our previous wars that that is exactly what happened. Although it is taboo to think that the powers that be looked the other way when the terrorists planned to carry out their attacks and or added and/or helped them in some way, it isn't too hard to imagine that after the cold war ended that people who made huge money on it saw the writing on the wall and secretly hoping that some new threat would come up to justify ramping up military spending again.
I can also point out various nuisances of the events that transpired on 9/11 that don't exactly jive with what we are told to believe, but I want to first heard what other forum members might have to say about this.