During the London Olympics last year RAF Northolt had several fighter jets ready to fly across the City from NW to East London and engage or shoot down any hijacked planes, this was common knowledge, there were six sites around the Olympic village with surface to air missles also.

http://www.independent.co.uk/sport/olympics/snipers-missiles-and-fighter-jets-over-london-get-ready-for-a-1bn-olympic-ring-of-steel-7697856.html

Anyone trying to say that the American military was unable to protect New York City and the Pentagon for nearly 2 hours (conveniently) on the day of their worst ever "terrorist attack" needs to read above, then do one