You guys seem to need a lot of complicated explanations. Here's a simple one - you invade someone else's country, you get a partisan resistance. Your complex explanations seem to ignore the simple fact that all the Iraqi factions except 1, the Kurds, are killing our soldiers. Like the simple explanation says, you've got partisan resistance. A lot of it is provided by people whose kids or parents you have killed. If it was my counttry, I'd do the same.
There is nothing new about wars of occupation. They all turn out this way, and they are all fueled by hatred and rage towards an invader. Human nature, not complex military forces are at work here, my friend.