Lose-lose situation. If the Shiites lose, Iraq is a terrorist state. If they win, Iraq becomes a client state of a terrorist state.
Sunnis are the natural rulers of Iraq even though there are more Shiites due to higher birthrates. Sunnis have always had their act more together and were more modern. Also seem smarter in that area. I’ll bet that if you looked at what Iraqis got university educations in Europe and the US you would see mostly Sunnis
The only thing that can alter this natural Sunni domination is Iran.
This Sunni-Jihadist takeover of Mosul and other cities is a reversion to the norm of Sunni rule. It is a rebellion against the Shiite central govt of Maliki
Win-win if they exterminate each other.
Shades of Vietnam invading Cambodia to get rid of Pol Pot.