Arif Khokar
I suspect the reason why you did not see a major problem prior to the US invasion was mostly because the press (internationally) barely paid attention to it.
Ethnic division was a constant problem in Saddam's Iraq. And not just between Sunni and Shi'a and Kurd, but within sects in each. The way Hussein delt with the problem was vicious and well-documented. But systematic attacks by security forces, most notably but not always, in southern and northern Iraq does not begin to tell the story.
Just as damaging, and more so in the long term, Hussein's more subtle policies of repression, including draining the southern marshlands, issuing edicts that prohibited Sunni's from engaging in commerce with the Shi'a, and the forced relocation of ten's of thousands of non-Sunni Iraqis, to name a few, resulted in a regime induced famine that end thousands. I won't even get into the diversion of Oil for Food funds and the seizure of humanitarian supplies.
Most people in the west did not recognize the extroardinary abuses in Mao's China or Pol Pot's Cambodia, again to name two examples, until quite some time after the fact. This despite the fact that there was plenty of evidence to suspect something "serious" was happening there.
John Teague