Religion my friend, Religion.
One Shia group will not piss off another Shia group.
The Iraq Iran war was Sunni, Shia thing.
One of the biggest realizations that has happen in Iraq is that Shia's are now more powerful then Sunni's, these groups look to be slowly learning to live together without fear of death squads or tyranny.
Who knows if it will work, but if it does, Bush's legacy will be a lot different in many parts of the world then it is now.
There will be elections in Iraq in September or October, this will be first real litmus test of the new Iraq, because all regions will vote.
Iraq is soon to become very wealthy as there southern fields become fully developed, and as more oil revenue comes back to the central government instead of being siphoned off by militant groups.