The Shia and the Sunni have been rivals since shortly after the death of Muhammad (whom both recognize as thier first prophet) when the two groups could not agree on who was teh rightful hier to Muhammad.  One group supported on person, the other group supported someone else.  As a westerner, I can never truly understand the ethnic tensions between teh two groups, or why in some cases they live in peace while in others, they fight each other.  In Iraq the tensions relate to more modern times.  Although the country of Iraq is mostly Shia (60-something percent), during the reign of Saddam Hussien (who is a Sunni), the Sunni held th epowere and badly mistreated the Shia population.  Once saddam and his regime were removed from power, the Shia began to retaliate for the decades of persecution.  The Sunni (in the government, at least) were ruthless and brutal to the Shia Maojority- of course the Shia are angry now.  Unfortunately, thier retribuative actions against the sunni have only helped to increase teh violence between the two groups.  Until teh Sunni and teh Shia start to work together, Iraq is going to be locked into a downward spiral.  The trick to sucess in Iraq is that the Iraqis have to learn to live together.  No one else can make them do that, certainly not America.