This article is from a person (who seems like he) is in the know.
Source: "Finally, no matter when or how the Saddam Hussein dictatorship eventually fell, there has long been a high probability that this would result in a civil war between Iraq's Shi'ite and Sunni Arabs. No minority ascendancy class gives up power willingly, especially where their power has been largely underwritten by violent repression of the majority over a long period. The insurgency in Iraq is mainly fuelled by Sunni fears of what will happen to them when the Shi'ites take charge and perhaps revenge.
Even critics of the US-led intervention in Iraq should be willing to admit that it is better that such civil strife is supervised and ameliorated by the international community rather than the Iraqis just being left to get on with a civil war unmolested."
The entire thing lays out the current situation and geopolitical parties involved in current rebuilding of Iraq very well.