As Cody of The Alternate History Hub predicted, the "War on Terror" here would be fought using drones, covert aid, and special forces insertion. Not big deployments.
I guess the U.S. military was also downsized after the Cold War, since the threat of the USSR no longer existed and China being marked as the OPFOR once more was not yet really a thing of concern.
To quote the book
102 Minutes: The Unforgettable Story of the Fight to Survive Inside the Twin Towers by Jim Dwyer and Kevin Flynn (page 281):
"On the morning of September 11, 2001, the U.S. military had fewer soldiers, sailors, marines, and aviators in its ranks than any time in the previous history. American governments had been engaged in overt and covert wars almost nonstop since 1941, and at the beginning of the twenty-first century the United States was still enjoying a peace dividend from the end of the Cold War. The 9/11 attacks roused a spirit of service and duty that bad been embodied that day by the valor of firefighters, medics, and police officers. Military recruitment surged. Over the next decade, two million members of the U.S. armed forces would be deployed to Iraq and Afghanistan."