Bush did NOT know there was difference between Sunni & Shiite Muslims until Jan '03
January 2003 the President invited three members of the Iraqi opposition to join him to watch the Super Bowl. In the course of the conversation the Iraqis realized that the President was not aware that there was a difference between Sunni and Shiite Muslims. He looked at them and said, "You mean...they're not, you know, there, there's this difference. What is it about?"  It's a bit like the U.S. president intervening in Ireland and being unaware that there are two schools of Christianity - Catholics and Protestants.

Eyewitness to a failure in Iraq
On April 16, 2003, a mob attacked and looted the Iraqi equivalent of the Centers for Disease Control, taking live HIV and black fever virus among other potentially lethal materials.  About the same time, looters entered the warehouses at Iraq's sprawling nuclear facilities at Tuwaitha on Baghdad's outskirts. There was nothing secret about the Disease Center or the Tuwaitha warehouses.  This equipment could be anywhere. But one good bet is Iran, which has had allies and agents in Iraq since shortly after the US-led forces arrived.

Divided, Iraq Can Stand:  An interview with former ambassador Peter Galbraith 
The Sunni insurgency can only be defeated by the Sunni Arabs. The constitution allows them to form their own region and have their own military. A Sunni Arab regional government and regional military may be able to win enough support to take on, or co-opt, many of the insurgents. An Iraqi Army loyal to a pro-Iranian Shiite government (and led by Shiites and Kurds) will never be seen as a national army by the Sunni Arabs.