While September 11, 2001 is well known as the day when the U.S. suffered its worst terrorist attack, what is little known is that it was also a day when large sections of the armed forces around the nation had been preparing to fight a simulated nuclear war, as part of major training exercises being conducted at the time. In their annual exercises "Vigilant Guardian" and "Global Guardian," the North American Aerospace Defense Command (NORAD) and the United States Strategic Command (Stratcom) were scheduled to carry out what has been described as a "simulated air war," a "full-blown nuclear war" exercise, a "fictional nuclear war," and a "practice Armageddon."
No official attempts have been made to fully investigate these exercises and what effect they had on the military's response to the 9/11 attacks. But evidence indicates they caused at least some confusion over what was "real-world" and what was simulation, and they may also have been a factor behind the communication problems experienced by military personnel that day. Other evidence suggests that some actions that have been presented as reactions to the terrorist attacks may actually have been related to these exercises--actions such as raising the alert status of American armed services to Defcon 3 and closing the huge "blast doors" to NORAD's operations center in Cheyenne Mountain, Colorado. There is also evidence that other "practice Armageddon" exercises were being conducted at the time of the 9/11 attacks, but details of these are unknown.
AMERICA'S AIR DEFENDERS WERE SET TO FIGHT A 'SIMULATED AIR WAR' ON SEPTEMBER 11
Perhaps the most important exercise to consider is NORAD's exercise called "Vigilant Guardian." Close examination of this exercise is imperative due to the crucial role NORAD had to play in responding to the 9/11 attacks.
It has been established that at the time the 9/11 attacks began, the United States Strategic Command (Stratcom) was in the middle of a major annual training exercise called Global Guardian. Stratcom is responsible for the readiness of America's nuclear forces, and the exercise aimed to test its ability to fight a nuclear war, being described as "one of many practice Armageddons" that the U.S. military routinely conducts. 
Questions arise over whether this exercise impeded the military--particularly the staff at Offutt Air Force Base near Omaha, Nebraska, where Stratcom is headquartered--in its ability to respond to the crisis. Global Guardian was not canceled until after 9:03, when the second WTC tower was hit.  In fact, some accounts suggest it did not end until after 9:37, when the Pentagon was struck.  So could military personnel have mistaken reports of the real attacks for part of the exercise? And might vital resources that could have helped stop the attacks have been unavailable, being used instead for the exercise?