The area that is now Iraq has never had the qualities of a nation. Great Britain created ill-conceived "national" maps of the area after World War II, grouping natural enemies and dividing regions of common ethnic and religious interests. Shiites and Sunnis have been at each other's throats for hundreds of years.
The war stated in 2003 by Bush and the Neocons cost thousands of lives and billions of dollars without fixing ANYTHING. We have stayed for decades and the result would have been the same.
Sending our troops and resources back in there would be like willingly jumping back into a pool of quicksand after just having escaped from it.