Lear said:
If a country does manage to invade the U.S., the U.S. is probably screwed. The reason is because the generals in the U.S. Military are rather incompetent. They forgo the tried, true, and smart tactics outlined in Sun Tzu's "The Art of War" that were used by them as recently as WWII. Why do you think they're failing in Afghanistan? It's not the troops. It's the generals. No amount of armed civilians, high tech weapons, or numbers can make up for incompetence in the highest ranks of command.
Sun Tzu wrote about defeating armies belonging to actual nations. Antiterrorism is a different kind of fight entirely. The United States dominates completely when actual battles occur, but you can't very well battle suicide bombers casually walking around in public. All Sun Tzu says that could help in this situation is to gather intelligence, and the US been putting in quite a few resources into that front. Sun Tzu says nothing about helping the local population and preventing the indoctrination of more enemies. In fact, given that the US has quite the arms stockpile, taking Sun Tzu's advice would probably involve blowing up the entirety of every country that houses terrorists. Not a very good idea, is it?
Sun Tzu may apply to skirmishes, but in a nuclear age where the enemy doesn't clearly display his colors, we need a new manual.