The city of Mosul has been under ISIS control since June 2014. The Iraqi government together with the coalition forces is planning to retake the city, apparently in the next month or so.
However, according to retired US Army Colonel Daniel Davis, retaking Mosul could ignite a far bigger war in Iraq, this time between Shia and Sunni militia forces who are planning to defeat ISIS. According to Colonel Davis:
As reports continue to emerge from the liberation (once again) of Fallujah earlier this summer, yet again Shia militia are being investigated for large scale executions of Sunni civilians in the aftermath of the “liberation” of the city, as happened in Tikrit before.
Furthermore, there is no single command over Sunni and Shia militias which would supervise the attack to retake Mosul. Davis also states that once the city is taken over, there would be an urgent need for humanitarian assistance for the residents of Mosul. These risks could mean that:
“Separately, without more help than we’re giving now, the headlines that are now dwelling on the humanitarian disaster in Syria, especially Aleppo, could easily shift to Mosul. Europe could suffer a whole new refugee crisis. That was another issue stressed by both Minister Mustafa and General Yassin. “If significant changes are not made before the battle begins, we will have a humanitarian catastrophe,” Mustafa said.
You can read the rest of Colonel Davis’s assessments from his recent trip to Iraq here.