What happened in Mosul when ISIL went in is that first they called all the archbishops, the Chaldean Syriac Catholic and the Syriac Orthodox archbishops, to meet with the ISIL leaders to come up with a solution. Basically, the bishops refused because they already had left Mosul.
A few days after that they came up with the fatwa that you either convert to Islam or leave.
A few weeks later they came back and said that, no, Christians can come back and they can pay the jizya. But already the damage had been done and people had already left.