Uh, try at least as early as the Sassanid Empire of the 3rd century. Later, various Muslim empires, then Mongul, then Muslim again, then British. They can and have played nicely together.
But in the present case, the deck is stacked and the situation has to explode:
1) Under Hussein, the Sunnis dominated and controlled the country, even though they are not the population majority. The Sheeites are the majority, and they want control.
2) The Sheeites are demanding that the government be set up through popular elections because they know they will win, the Sunnis are fighting very hard now to make sure that doesn't happen.
That unstable structure (where a minority group held down the majority group by force) will cause an explosion when we pull out. The Sheeites have payback in mind for what was done to them under Hussein, and the Sunnis know it.
What's worse, the major political power in the country in all factions is held by religous leaders and they are demanding the government be based on Islam. The US will never allow this because we just had top go into Afghanistan to kick out one of those.
I don't know how that place could be any more screwed up. Every side is demanding and saying something different, there seems to be no middle ground, and Bush just keeps saying "we'll be handing you the keys in July".