According to For U.S. Military, A Key Iraq Mission Is Averting Civil War, Sunnis and Shiites in largely Sunni Tal Afar got along and intermarried until the U.S. invaded then pulled out.
At that point the Sunnis feared a Shiite takeover and invited religious extremists in from Mosul and Syria:
In May 2005, not long after he arrived in the city, Col. Hickey sat in on the interrogation of a 17-year-old member of a Sunni assassination cell. Under questioning, the boy explained that his job was to hold the legs of the victim while someone else lopped off the head. When the interrogator asked the boy what he aspired to, ‘he responded that he wanted to be the guy who got to cut off the head,’ Col. Hickey recalls. ‘It was chilling.’
The Shiites responded by taking over the police force and turning it into a death squad.
And now Col. Hickey is trying to mediate between the two groups.