Pygalgia gives a link to Juan Cole. While there I came upon this link that summarises the situation nicely.
Yes and I do realize that the situation is more complex than just Sunnis against Shias. And the U.S. has not made this situation better. And in addition to the above article there is one other factor that is basic and has been totally overlooked. A factor more important than oil, money, clothing, housing, food and even water. That factor would be respect. Without it people will do anything to attain it including forgoing food and water. I think that was proven when Pelosi went to Syria and actually was able to accomplish something. Respect and diplomacy really do work.