the US has never at any time had sufficient forces on the ground to be able to take control. They needed twice as many troops after the fall of Bhagdad and needed to immediately impose order.
Instead they just continued fighting sporadic groups using the forces that penetrated and defeated the 'mainstream' troops defending Bhagded - whilst entirely neglecting civil unrest and disorder.
The price is now being paid for winning the 'prime time' war.
Nowadays, nearly all of the US effort is spent in merely defending its own ground troops. No impact whatever is being made on the multitude of disparate interests fomenting anarchy - those interests are only concerned with grabbing as much power as they can for themselves and clearly the idea of national unity at this stage is not even on the table.
What a mess, and what a huge mistake the US made in ever invading Iraq. The US were responsible for thinking ahead in terms of what was likely to happen post invasion, and they clearly never did. They were too concerned with rubbing Hussein's nose in it to consider the aftermath.
It will be many years before the hole called Iraq ever even approaches a semblance of unity. And I feel really sorry for all those Yankee soldiers dying for pr!cks like Bush, Cheyney, and Rumsfeld.... but I hear Bush's golf handicap has improved during the course of the war....