I personally think metaphors are one of the best tools to use to apply global politics to everyday life. If you don't agree, I'm sorry, but I use them as a tool. OK, let us assume we find a country who aided the terrorists resoponsible. Those actions would be seen as an attack on the United States, and war would ensue, with all of NATO aiding the US. After the war, the country responsible would probably have its current government stripped, a new, democratic government would be set up, and a peacekeeping force of NATO and UN military forces would be installed. After a few years, the country should be able to choose their on government and move on.
Let us now assume that we find a country who knowingly harbored those responsible. The first thing we would do , would be to ask for an official apology, possibly reparations, and the promise that either A: The terrorists are brought to justice by the local government or that B: The US is allowed to bring them to justice. If my earlier threads implied that we would go to war regardless, that is because I doubt that if we come across situation B, the country in question would give an apology/reparations.