Sunnis and Shia have been at each other's throats for a LONG time. The only reason why you see so much violence between them now as opposed to before this war is that Saddam reined with an iron fist, and no Sunnis were harmed due to the nasty repercussions that would follow. Same thing as with all the ethnic nastiness that followed the breakup of Yugoslavia following the exit of the Soviets. They were once again allowed to slaughter each other.