Not so much now. If you try to do something on a plane, people jump on you. That's what thwarted both the shoe and underwear bombers, and that's what would thwart another attempt at ramming planes into buildings.
One of you will end up here next!
We have more guns than people here. Haven't you people seen RED DAWN?
but as to that note...I think thats a pretty good reason why it is unlikely we will ever be invaded. Forget about fighting through the guard...watch some foreign army try to fight their way through Compton...