Post Victory....things were going fine for part of a day.....but when that Genius Paul Bremer fired the entire Iraqi Government, stopped all retirement pay, and decided to clean house of all Bathist Party Members.....well shall we agree things went to Hell in a Hand Basket?
Jay Garner, much like George Patton following WWII, was going to continue to man the Iraqi Military, Police, and other government agencies using the existing personnel.....and gradually removing those culpable in crimes against the Iraqi People during the Saddam Regime.
He got replaced and Bremer showed up and took charge.
That was the cause of the Insurgency pure and simple.
It was not as much lack of planning....it was an over abundance of stupidity....something the American Government seems to have an excess of no matter which party is in power.