The exercise Operation Vigilant Warrior may have been a significant factor in the success of the 9-11 attacks.
We'll examine this case as it's portrayed in "The Military Drills on 9-11: "Bizarre Coincidence" or Something Else?", by Four Arrows (aka Don Jacobs).
The Center for Cooperative Research has an extensive time-line relating to the military exercises of 9/11 with ample documentation on a number of them (See footnote 15 for the web address). One of the drills discussed is ‘‘Global Guardian’’ that was scheduled for October 2001 but apparently was rescheduled for early September. According to the Center’s research, Stratcom may have incorporated a computer network attack into Global Guardian with the claimed ability to actually shut down its own systems. It is not known if this occurred or what the effects might have been on the air defense system, but it is another item that demands further inquiry.15 The existence of these drills and others, like ‘‘Amalgam Virgo,’’ which is discussed later and others yet unconfirmed, begin to paint a picture that is beyond coincidence.
"The Military Drills on 9-11: "Bizarre Coincidence" or Something Else?"
The current Cooperative Research page on Global Guardian has changed a little since he wrote this, presumably. Visit the link and you’ll see the exercise was running on 9/11, although it wasn’t just a single day thing (it had started the previous week).
The computer network attack scenario, however, remains as speculative as Jacobs suggests:
A 1998 Defense Department newsletter reported that for several years Stratcom had been incorporating a computer network attack (CNA) into Global Guardian. The attack involved Stratcom “red team” members and other organizations acting as enemy agents, and included attempts to penetrate the Command using the Internet and a “bad” insider who had access to a key command and control system. The attackers “war dialed” the phones to tie them up and sent faxes to numerous fax machines throughout the Command. They also claimed they were able to shut down Stratcom’s systems. Reportedly, Stratcom planned to increase the level of computer network attack in future Global Guardian exercises. [IAnewsletter, 6/1998 ] It is not currently known if a computer attack was incorporated into Global Guardian in 2001 or what its possible effects on the country’s air defense system would have been if such an attack was part of the exercise.
It’s an extra exercise, then, but there’s again no evidence for it having a negative effect on response to the 9/11 attacks.