They could (and should) have kept the Iraqi military infrastructure intact, instead of completely disbanding it. They could have weeded out the Saddam loyalists and kept the people who could be of use. They could have done a much better job of winning the peace. Much of the problems with insurgents stems from the way Iraqi civilians have been treated (very badly) by the US military. Many things could have been done to show respect for the people of Iraq that were not, such as securing the places where Iraq's national treasures and historic artifacts were kept, instead of allowing them to be completely ransacked. There are just so many ways in which the US government and the Pentagon in particular have shown absolute and utter contempt for and to the Iraqi people.

These things are just as much, if not more responsible for the state of affairs in Iraq as anything else. And combine that with the fact that the US government is not delivering on the reconstruction money it promised and the fact that people aren't getting their basic needs met, and you couldn't have a better recipe for the kind of problems that we face in Iraq right now.

No, this was done on purpose so that the US would have an excuse to keep its military in Iraq indefinitely.