Slaughter of millions of Iraqis,
Destruction of the remaining infrastructure, invasion by Iran, the creation of a ME islamofascist superstate, the nuking of Israel and likely parts of Russia and Europe and the US. and an attempt (likely successful) to take over Saudi Arabia, the Gulf States, Jordan, and roll in Egypt and N. Africa in the superstate's sphere of influence.
Oil at $100, and finally a much much more costly American led solution to what is now being kept as a very manageable and improving situation.
the costs to us will be: economic disruption, our youth fighting a world war in our millions, US casualties in the 10X's of thousands if not much higher.
Ed