the military systems didnt fail.
the hijackings were detected early, but they were waiting for demands, as was the usual MO for hijackings. nobody expected the planes to crash into the WTC, and nobody was gonna just smoke a planeload of passengers when it was most likely gonna be a hostage situation.
your hindsight is flawless but your logic is flawed
would you preferr computer controlled automatic missile launches for every plane that deviates from it's course? or poison gas canisters that flood the cockpit every time a collision warning goes off?
normal fallible people made judgement calls on that day which look stupid from the perspective of history but thats not proof of a conspiracy, or incompetence. If fucking up is all thats needed to become part of a sinister plot to destroy america then Obama is the most fiendish mastermind since Migelito Loveless.
Donald Rumsfeld had this to say:
"Secretary of the Army Tom White, who has a responsibility for incidents like this as executive agent for the Department of Defense, is also joining me."
Know when Tom White became the executive agent? on 9/10/01, know who promoted him? Don Rumsfeld, fucking scapegoated him is what he did.
CJCSI 3610.01, dated July 31, 1997, required that all requests for assistance in hijackings be approved by the Secretary of Defense. An update to that order, CJCSI 3610.01, dated June 1, 2001, had an exception for emergencies that would seem to give commanders in the field autonomy in ordering intercepts. However, that exception did not cover requests for "potentially lethal assistance", the kind required to respond to the attack:
(DODD 2025.15, Feb. 18, 1997) 4.4 The Secretary of Defense retains approval authority for support to civil authorities involving: use of Commander in Chief (CINC)-assigned forces (personnel units, and equipment) when required under paragraph 4.5, below; DoD support that will result in a planned event with the potential for confrontation with specifically identified individuals and/or groups or will result in the use of lethal force.
DC is the most protected airspace in the world, yet the Pentagon suffered a direct hit, apparently by a plane that had no engines.