The story...

The exercise Operation Northern Vigilance may have been a significant factor in the success of the 9-11 attacks..

Our take...

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 Vigilance’’ was a third exercise planned for 9/11. For that, according to Alex Jones and Paul Joseph Watson in their online article, ‘‘Wargames Were Cover For the Operational Execution of 9/11,’’ on the morning of 9/11, jets were removed from patrolling the U.S. east coast and sent to Alaska and Canada.8 This is confirmed from ‘‘the horse’s mouth’’ in a newsroom release directly from NORAD on September 9, 2001 entitled ‘‘NORAD maintains Northern Vigilance.’’ It states, ‘‘The North American Aerospace Defense Command shall deploy fighter aircraft as necessary to Forward Operating Locations (FOLs) in Alaska and Northern Canada to monitor a Russian air force exercise in the Russian arctic and North Pacific Ocean.’’9
"The Military Drills on 9-11: "Bizarre Coincidence" or Something Else?"

This at first sounds like confirmation of something important, but let’s take a look at the details. Here’s what the “Wargames Were Cover...” article, cited here, says about Operation Northern Vigilance:

1) OPERATION NORTHERN VIGILANCE: This was planned months in advance of 9/11 and ensured that on the morning of 9/11, jet fighters were removed from patrolling the US east coast and sent to Alaska and Canada, therefore reducing the amount of fighter planes available to protect the east coast.
http://www.prisonplanet.com/articles/september2004/080904wargamescover.htm

So was this really confirmed by the quote from NORAD? Well, no.

The article says “jet fighters were removed”, for instance. The NORAD quote says fighters “shall be deployed as necessary”. Was it necessary? Perhaps, but the quote doesn’t say.

Then we’re told the fighters would be removed “from patrolling the US east coast”. NORAD doesn’t say where the fighters would be taken from, or what (if anything) they would normally have been doing.

Finally it’s claimed that this would reduce “the amount of fighter planes available to protect the east coast”. If this is intended to imply that NORAD would have had more fighters on alert had the exercise not been running, though, there’s simply no evidence for that. NORAD reportedly had only 14 fighters on alert on 9/11, and an article in Airman (the “magazine of America’s Air Force”) from 1999 confirmed the same thing. That is, this was the standard number available:

The Air National Guard exclusively performs the air sovereignty mission in the continental United States, and those units fall under the control of the 1st Air Force based at Tyndall. The Guard maintains seven alert sites with 14 fighters and pilots on call around the clock. Besides Homestead, alert birds also sit armed and ready at Tyndall; Langley AFB, Va.; Otis Air National Guard Base, Mass.; Portland International Airport, Ore.; March ARB, Calif.; and Ellington Field, Texas.
http://www.af.mil/news/airman/1299/home2.htm

Look at the apparently definitive claims more closely, then, and they reveal a distinct lack of supporting evidence. There’s no compelling reason given to believe that Northern Vigilance had any effect on the outcome of 9/11.