Then we must not have our pants down. If such attack hppens we use our own arsenal of weapons and completely destroy the offending nation. Scorched earth. Such as in WW2 when we had the atomic bomb, we had 2 of them. I would have used one on Tokyo and the other on Berlin. With Osama Bin Ladin and Afgansitan I wouldn't even bother sending in ground troops. Simply wipe Afganistan off the map, leveling it.
If they want to attack us then we have to teach the world that we will not hold back and send in soldiers. We will use the strongest most capable weaponry we have in our arsenal.