The Sunni backlash against Shi’ite government rule began in Hawija, near Kirkuk, way back in early 2013. The Hawija massacre led to Sunni protests and growing violence, which ISIS used to insinuate itself in Anbar and eventually to take almost a third of the country over.
Today, ISIS is in charge of Hawija, and reportedly executed 19 members of the Ubaid tribe, one of the largest Sunni Arab tribal factions in the region, after the tribesmen refused to swear allegiance to them.
Kirkuk officials claim the killings are fueling “widespread anger” against ISIS rule in the town, though it remains unclear whether this was an isolated incident with a few tribesmen, or another large-scale tribal blowup.
ISIS has tried to keep the Sunni tribes in its territory, both in Iraq and Syria, more or less on its side, and has been pretty successful in most of the regions. The few times they have faced tribal opposition, they’ve responded with brutal crackdowns aiming to dissuade anyone else from being the next to rebel. ISIS probably can’t hold off a full-scale tribal rebellion at this point, however they probably don’t need to, as fear of a return of Shi’ite militias and the Iraqi government is keeping many Sunnis on their side.