The insurgents are just doing what any people would do if they were invaded and occupied…
First off I agree with the rest of your post and your stance on the war. I just wanted to point out that this particular sentence is the wrong argument to use to convince someone else of that. The insurgents are not doing what any people would do if they were invaded. If the U.S. was occupied by Russia, I wouldn’t go out and start blowing up Buddhist temples in the name of Christianity, I would blow up Russian military targets.
The insurgents want to appear to be rebelling against the occupation and the Iraqi government, but their actions say otherwise – they are killing people of other ethnicities and beliefs simply because they differ from their own. The U.S. occupying forces just happen to fit into that extremely broad category, and the invasion was an opportunity and excuse for them to start exercising some of the hate and ignorance that consumes them. They’ll happily keep killing after we withdraw.