An unlikely array of forces is converging on the city of Mosul, lining up for a battle on the historic plains of northern Iraq that is likely to be decisive in the war against the Islamic State group.

The usual alliance - Iraqi troops alongside Shiite militiamen, Kurdish fighters, Sunni Arab tribesmen and US Special Forces - underscores the importance of this battle. Retaking Mosul, Iraq's second-largest city, would effectively break the back of the militant group, ending their self-declared "caliphate," at least in Iraq.

But victory doesn't mean an end to the conflict. In a post-Islamic State Iraq, the rivalries among the players in the anti-IS coalition could easily erupt.