Mass Effect & Mass Effect 2
At about the time the salarians and asari were forming the Council, the turians were embroiled in a bitter civil war. The Unification War, as it was later named, began with hostilities between the colonies furthest from the turian homeworld, Palaven.
These colonies were run by local chieftains, many of whom had distanced themselves from the Hierarchy. Without the galvanizing influence of the government, the colonies became increasingly isolated and xenophobic. Colonists began wearing emblems or facial markings to differentiate themselves from members of other colonies and open hostilities became common.
When war finally broke out, the Hierarchy maintained strict diplomacy and refused to get involved. After several years of fighting, less than a dozen factions remained, and the Hierarchy intervened. By that time, the chieftains were too weak to resist; they were forced to put an end fighting and renew their allegiance to the Hierarchy.
Though peace was restored, it took several decades for animosity between colonists to fade completely. To this day, most turians still wear the facial markings of their home colonies. As a point of interest, the turian term 'barefaced' refers to one who is beguiling or not to be trusted. It is also a slang term for politicians.