The U.S. forces leaving Iraq isn't going to stop the violence or even slow it for that matter. Theres an ancient feud going on between different sects and it's never going to stop unless a government is set up that makes a compromise to satisfy all warring parties. It's true there wasn't as much of this before the U.S. invasion, the invasion brought anarchy as well as chaos, but now that the U.S. has crushed the old system it has to finish what it started, for the U.S. to leave right now would just leave that area chaotic and helpless. Bush should of left them alone in the first place, they had more peace with Saddam in control.
The U.S. isn't the cause of the violence and chaos, the U.S. just broke the levee thats all. Now that they fucked with it they gotta build a new and even better dam, it's your obligation to finish what you started.