What does G.W. Bush have to do with the civil war in Iraq between Sunni and Shi'a?

Please explain what President Bush of US has to do with the Iraqi Civil war between the opposing Sunni and Shi'a. I mean he didn't start it, right? As I know, the Sunni and Shi'a have been opposing each other for quite some looong time now.. I don't know.. correct me if I'm wrong or something. Please I need it for a cl report T_T thank you very much. And if possible, may you also add some infos about the Civil War. Thanks again ^^,