Why withdrawing US forces from Iraq will promote greater violence...against those forces.
Those that are the true trouble makers in Iraq (Okay, for those of you that will say that the trouble makers are the US forces, I stick my tongue out at you.) will ramp up the violence when it is apparent that US forces are truly leaving. They want us there.
Not only do they get to dispatch a few US troops to the hereafter on occasion, they also get to wallow in the orgy of pleasure provided to them because the war is an emotional open wound in the American psyche. They profit from our population being in conflict with one another, and they want that open wound to continue to fester.
I will not argue against pulling our troops out of that tragically flawed area of the world, however, I will point out that the process will be violent. Especially towards the end.