Consider the precautions taken by Italian authorities when warned by the CIA that OBL and his boys might try to crash a hijacked plane into the G8 summit. The Italians came up with the positively ingenious idea of ringing the conference site with air defense assets, things like AAA, SAMs, and the Italian Air Force. The Secret Service, meanwhile, made Pres. Bush spend his nights on an aircraft carrier.

The G8 conference was set for a specific date and location. Neither was known in advance for 9-11. Duh.

As for deploying Stingers around the Pentagon, whether an hour's warning is enough depends on where the Stingers are stored in relation to the Pentagon. As a headquarters building, it may not have an in-house armory. If not, then the Stingers would've had to be brought in from elsewhere. Doing so would first require people to realize that the two planes that struck the WTC towers weren't the only ones hijacked.