Everyone who has taken a fair look at the evidence has concluded that -- at the least -- the U.S. military was intentionally stood down on 9/11. There are numerous lines of evidence that certain high-level people within the U.S. military participated in the intentional stand down.
But what of the many rank and file employees of the various defense departments who were good men and women devoted to protecting their country? Why didn't they stop the 9/11 attacks?
Part of the answer is that they were cut out of the loop by the conspirators, and didn't have the information or access to be able to take preventative action. But another part of the answer is that they were bombarded with distractions, so that they could not focus on their job.
"Operation Northern Vigilance is called off. Any simulated information, what's known as an 'inject,' is purged from the screens".
In other words, someone had inserted false radar blips onto air traffic controllers' screens as part of the 5 or more war game exercises occurring at the time of the attacks (Vice President Cheney was apparently in charge of ALL of the war games and coordinated the government's "response" to the attacks; see this Department of State announcement; this CNN article; and this previously-cited essay).
And air traffic controllers claim they were still tracking what they thought were hijacked planes long after all 4 of the real planes had crashed. This implies that false radar blips remained on their screens after all 4 planes went down, long after the military claims they purged the phantom war-game-related radar signals
As a second example, fighter jets were also sent far off-course over the Atlantic Ocean in the middle of the attacks (testimony of Senator Mark Dayton), neutralizing their ability to intercept the hijacked airliners. To this day, no one has admitted being the person who sent the aircrafts on a fools' errand.
A former air traffic controller, who knows the flight corridor which the two planes which hit the Twin Towers flew "like the back of my hand" and who handled two actual hijackings says that that planes can be tracked on radar even when their transponders are turned off, and that Donald Rumsfeld and the Pentagon tracked three of the four flights from the point of their hijacking to hitting their targets (also, listen to this interview). Therefore, it is very odd that aircraft were sent off to chase phantoms over the Atlantic.
As a third second example, NORAD and other employees trying to defend the nations' skies on 9/11 were inundated with massive, simultaneous phone calls. While it is still unclear, it is possible that the barrage of phone calls was part of a coordinated terror drill and a pre-planned distraction. Further research needs to be done to verify whether or not this was the case.
As the above examples show, the honorable rank and file military people trying to protect the U.S. on 9/11 never had a chance.