If anyone invaded the USA, they could not win.
Because we would keep shooting them until they left. That is what happened in Vietnam, and that is what is happening in Iraq. End of story.
We won WWII because we were kicking invaders out of other countries. It just so happened that by the time we fought our way to Germany, they had lost their entire infrastructure and the will to fight, and Hitler's inner circle was starting to break up. The German people had no beef against the Allies, they were spurred into it just like we were in Iraq.
We won in Japan because of the bomb and because they had no infrastructure left. The Japanese bought the war effort because they were led into it by a fascist military that had hoodwinked the Emperor. Look at Germany and Japan now, they want nothing to do with war. They learned, will we ever learn?
More in the foxhole.
We ultimately lost the ideological wars in Cuba and the Philippines too, and had to leave. We had our asses handed to us in Lebanon, so we had to go bully the little kid next door (Grenada). WWI Germany and Napoleon lost wars of occupation. Spain lost the new world because they eventually became the occupiers, and so on.
There is nothing that the US can do to leave after the shooting stops, because it won't. If Bush defines winning as Sunnis and Shiites living next to each other and both of them waving at each other and to the US Humvee patrol driving down the street in the morning while they pick up the paper from the front porch, it'll be a long wait. Especially if that paper has to say all is well and the coalition Sunni - Shia - Kurd government is all happy and the state coffers are full from oil money. If they are waiting for the neighborhood to invite a US tank to visit the block party so kids can climb on it during Iraqi independence day, my son will be worrying if his son is going to get his ass blown off in the Middle East, and I can't say which country, because if we are still there in 35 years, we will be fighting everyone.
It won't stop until everybody leaves them alone and they have their uncivil war. Eventually Iraq has to learn that shaking hands is better than pulling triggers, but that is only a self learned lesson.
If we should be going in anywhere, it is to Darfur, and to make it an independent country, but we should go only as 40% of an African UN coalition. We should go in to save lives, not to topple anti-democratic governments. If we were to do that, we'd need to be in about 10 places at once. And yes, Asshat Bush would do that if he could, but not in the right places or for the right reasons.
There is legitimate use of US military force on foreign soil. Haiti under Clinton, WWII, stopping piracy on the open seas, disaster relief efforts, (maybe we could consider using US troops in the USA to help after devastating hurricanes, or not, it's an idea).
Finally, were we invaded, by an army or by guerilla terrorists, an able Army and National Guard along with an up to date CIA and FBI would do some serious bud-nipping. This shit has got to stop, and progressives in the USA are the ONLY ones that can do it. Only the Iraqis can stop this war, we can't.
What we can do is help create the conditions so they can. Diplomatic and relief efforts, but no infrastructure work until the shooting stops. The Iraqis need their Ghandi. Let us be Ghandi for them until they have their own. So help the Iraqis help themselves: subpoena, investigate, embarrass, impeach, but STOP THE WAR.