Why did US Military not act during the time of the hijackings and plane crashes ?

Stand Down

Orders deter the US Military from intercepting hijacked planes on 9/11 – EXPOSING THE NORAD WAG: The 911 Window Dressing Tale Using NORAD’s Own Press Release And Fifth Grade Math, Published January 8, 2003

9/11 USAF Stand Down

NORAD was unusually prepared on 9/11, because it was conducting a week-long semiannual exercise called Vigilant Guardian.
On 9/11, North American Aerospace Defense Command's (Norad) Northeast Air Defense Sector (NEADS) was fully staffed, its key officers and enlisted supervisors already manning the operations center "battle cab." [Aviation Week]
COLONEL ROBERT MARR, US AIR FORCE: We had the fighters with a little more gas on board. A few more weapons on board. [...] We had 14 aircraft on alert, seven sites, two aircraft at each site. [ABC News]  That's a ratio of 3.5 'hot' fighter jets per hijacked airliner.

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