By Ali Allawi, former Iraq Minister of Defense is authoritative, and comprehensive.
to Quote Allawi, "the invasion of Iraq tipped the scales in favour of the Shia, who are now determined to emerge as the governing majority after decades, if not centuries, of perceived disempowerment and oppression." He goes on, "In the sterile world of zero-sum politics, the loss of power of the Sunni Arab community in Iraq was soon translated into a raging insurgency that challenged not only the US occupation but also the new political dispensation."
American Politicians might do well to pay attention to him.
This link will take you to an excerpt from Allawi's book on the Iraq situation. http://news.independent.co.uk/world/middle_east/article2332770.ece
It is also worth the time to read.