There was one of Jeremy Bowen's reports where it finally gelled for me about the Sunni/Shi'a thing but that memory got overwritten, so to speak.
Whichever group Saddam belonged to, they were smaller, numerically but the 'ruling class' - Iraqi equivalent of The Normans. We went in, dismantled their army, police and security services, train up replacements, move out, leaving everyone with a grudge to settle to get on with killing each other.
Why do the Sauds never lift a finger? Not allowed to kill fellow Muslims, presumably? Do errors of omission, or inaction, count?