I'm not sure if the root is necessarily in American consensus politics. Foreign policy is generally made and executed in the executive branch, and certainly Iraq policy for the last four years has been the sole purview of this Administration until Democratic Party Congressional victories a few months ago.

The problem, I think, was that the post-invasion planning for Iraq was unrealistic, unresourced, and generally unserious. Plan A was halfass, and Plan B didn't even exist and was tossed together on the fly, without even a proper chain of command or communication between the civilian and military elements. Rather than dictating a course we ended up responding to events, and the end result has been chaos, waste, and the current wonderful situation we have now. What was always going to be very difficult ended up becoming much, much worse than if proper grownups had been in charge.