Why didn't jets intercept the airliners since they had numerous warnings of terrorist attacks?

The Pentagon was hit 1 hr., 20 minutes AFTER the two towers were hit----why wasn't nothing perpared to stop it? Shouldn't the White House, Pentagon, Congress etc. all those goverment buildings been prepared once the 2nd plane hit, once they knew it wasn't a accident but a terrorist attack? Is our goverment's emergency plans THAT slow terrorists can hit another building after they've hit 2 buildings 1 hour and 20 minutes later and there is STILL no jets prepared to take it down?