point fingers at Americans for the excuse to kill your fellow countrymen. That makes sense!
Let's go back to the occupied USA example for a minute. What if the Democrats acquiesced to the occupiers and fully supported them, even going the distance to help set up a new form of government, one that was not a democracy. And on the other hand, we have the staunch Republicans who will not bow to any other form of government, let alone being occupied by another country. Radical Republicans start attacking the supporters of the occupying country - the Democrats. The Democrats start fighting back. Religion doesn't even factor into this chaos. It's Red against Blue. Even within some groups, there are Democrats who take the side of the Republicans in that they despise the occupation. They attack their own "people" for giving support.
Hypothetical situation or not, this would probably happen. In the USA, just look at how people on both sides are disparaging each other simply for supporting or denouncing the war in Iraq. The ones for the war call the pacifists "Anti-American" and "terrorist supporters." These are just words, but really, how far off are we from having our own little civil strife between these groups? When these group's emotions get fired up as it is and we don't even have an occupying force in our backyard, how safe do you think the streets would be if we did?
A similar situation is happening in Iraq right now and we were the cause of it. You have two distinct populations vying for power and control of the country. They're not killing each other for the fun of it. They both want control of the country and the US forces are stuck in the middle of one of the largest power plays in the history of man. The US might as well get the hell out of Dodge and let them fight it out. If not, this thing is going to linger for many, many more years.