The answer lies in the war objectives
As early as the 19th century, the great military strategist and historian of the Napoleonic wars defined war as "politics by other means". In other words war is not an end initself only the instrument throught which stated national/ politicals goals be achieved. This lesson of statecraft was entirely lost on the Bushmen who in their misguided arrogance and deluded by a false sense of infallibility decided to redraw the map of the middle east with a pliant US surrogate lording over the badland of Iraq. In order to put this neoconservative agends into action a justification was created using the Weapons of Mass Destruction as the foil to launch an aggressive international campaign against Iraq. We know the rest.
The planning for the war was inadequate and the military leadership completely misread the signals from the Arab streets. Of course in the heady days after the fall of Saddam Hussein there was jubiliation and had the US installed a puppet at that time and withdrawn both the USA and Iraq would have been saved a great deal of pain. Instead Paul Bremmer and other BUshmen felt that they could democratise Iraq by encouraging an Iraqi brand of identity politics.
The result of the policy of encouraging the Shiaas against the Sunnis is there for all to see. Sectarian violence has already reached epidemic propotions. The lancet has estimated that nearly 650,00 civilians have died since the Incvasion and a death toll of 200 daily is not shocking any more. The US invasion must be held responsible for this gigantic loss of civilian life.
The Iraqi constitution which is said to be based on power sharing between the Kuirds, Shiaas and Sunnis is only helping to perpetuate sectarian divisions. While it may not be fashionable to say so, the fact remains that with all the problems of governance associated with Saddam and the Baathist Party, the fact that it provided a viable ideological carapace for a highly divisive society.
The death penalty given to Saddam Hussein if carried out will lead to a virtual partioning of Iraq along sectarian lines and the world will have another failed state to provide a safe haven for Islamic terrorism.