Every Iraqi male over the age of 12 is ready to kill somebody.
Let them all kill each other.
It seems that way. Not just in Iraq either.
But if we just walk away and let them kill each other to their hearts content, we should consider what will happen if Iraq disintegrates. For starters, here's a hypothetical scenario that could turn out to be very real:
1. The U.S. pulls out, and Iraq disintegrates.
2. Iran and Turkey carve up the Kurdish part of Iraq, either by agreement between the two nations, or by fighting each other.
3. Iran and Syria carve up the rest of Iraq.
4. At that point Saudia Arabia, Kuwait, and the UAE would be unable to stand against Iran militarily on their own. Promises of protection by the United States would sound pretty hollow. Iran could either take them over directly, or they might reach an "accomodation" with Iran that had the same practical effect.
5. Iran would then control a huge proportion of the world's oil supply.