The exercise Operation Northern Guardian 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).
‘‘Operation Northern Guardian’’ may have been part of Vigilant Guardian, but related to simulating hijacked plans in different sector. Operation Northern Guardian involved deployment of aircraft from Langley Air Force Base to Iceland (Pevey, 2002). In late August 2001, 6 jets and 70 people deployed to Iceland for Operation Northern Guardian. Another 6 jets and 115 people deployed to Turkey to enforce the northern Iraqi no-fly zone. The members in Operation Northern Guardian in Iceland returned on December 3rd, 2001.11
"The Military Drills on 9-11: "Bizarre Coincidence" or Something Else?"
How does Jacobs come to assume Operation Northern Guardian has anything to do with hijackings? He doesn’t say.
Further, note that all we know about the dates here is that the exercise took place between “late August” and early December: there’s no indication that anything happened on 9/11. And even if it did, these fighters were in Iceland, so the act of carrying out the exercise is most unlikely to have affected NORAD in the US.
There’s still the argument that the fighters reduced the staff available at Langley on 9/11, of course. However, as they only needed to maintain two fighters on alert, it would be surprising if this caused any problems at all, especially for a location described as “one of the largest fighter bases in Air Combat Command”. Which means that, once again, there’s no real evidence to show this exercise had any effect on the 9/11 attacks.