On 9/11 there was no reaction from the USAF as hijacked aircraft flew through US airspace and plowed into buildings. This lack of response is inconceivable unless the USAF was stood down.Careful planning made this easy to achieve:
Chairman of the Joint Chiefs of Staff Instruction CJCSI 3610.01A (dated 1 June 2001) changed the protocol so that any requests for "potentially lethal support" had to come explicitly from the secretary of defense, leaving commanders in the field unable to respond to hijackings in any meaningful fashion.