Prior to and on 9/11/01, various government agencies routinely ran exercises and wargames anticipating 9/11-type attacks.
On the morning of 9/11, at least one wargame exercise simulated aspects of actual events as they took place. While this should have accelerated the military response, it did not.
Prior to 9/11, the military bragged about successful drug interdiction scramble intercepts. The military was able to find small jets trying to evade radar but on 9/11, without a cloud in the sky, they were unable to scramble fighter jets to get close enough to any of the four flights to intercept them.