Because of what the Pentagon calls, “bizarre coincidence,” on Sept. 11, 2001 NORAD was three days into Operation Vigilant Guardian. Held twice a year to tweak NORAD’s continent-spanning surveillance and interception web, North American air defenses that morning were aggressively alert and battle staffed, with key officers needed to make immediate decisions stationed in the "battle cabs" of each interlinked US Air Force command post.
http://willthomas.net/911/911_Commission.htm

We also know that despite the end of the Cold War and close ties with America’s new Russian ally, Operation Vigilant Guardian directed the attention of US air defenders to a simulated Soviet threat coming in over the North Pole.

Richard Ben-Veniste, one of 10 members of the 911 commissions, earlier pledged to pursue this suspiciously timed air defense exercise, “very, very diligently,"

And the former Watergate prosecutor did so, asking Gen. Arnold under oath on May 23, 2003:
“ Sir, given the awareness of the terrorists’ use of planes as weapons, how was it that NORAD was still focusing outward in protecting the United States against attacks from the Soviet Union or elsewhere and was not better prepared to defend against the hijackings scenarios of a commercial jet, laden with fuel, used as a weapon to target citizens of the United States?” [911 Commission Testimonies: Remarks of NORAD Personnel May 23, 2003]

Maj. Gen. Arnold admitted that “back to 1998” the Pentagon’s top brass were calling Osama bin Laden “the most dangerous man in the world. And our focus, with the demise of the Soviet Union Warsaw Pact, was that we felt like the greatest threat to the United States would come from a terrorist…or rogue nation.”

As Major Arias told the News Herald in June 2001, “The Cold War is over.” So who gave the orders for focusing US air defenses on the North Pole on Sept. 11?

5. Isn’t it true that America’s air defenses looking out for external threats were not expecting an attack from within the continental USA?
" It was initially pretty confusing," Gen. Myers later told the military press. "You hate to admit it, but we hadn't thought about this." [American Forces Press Service Oct23/01]