It's really not as simple as that. The reason a few hundred to a thousand men from the ISIS have been able to take Mosul is because it is friendly territory for the most part. The differing factions of Iraq can't seem to put their differences aside and come together. The Iraqi regular army had a minimal force in Mosul but they abandoned it when 'it' hit the fan. Taking an unoccupied city isn't really a huge success story in my opinion. That's like occupying an empty hole. There are cities where the battle will actually be fought and the citizens will rise up and defend their towns.
And this is why Crazy Uncle Joe may get what he has wanted for some time and that is a break up of Iraq into 3 distinct countries. The problem though, as I understand it, is that the Shiites and Kurds will both have oil producing territory. The Sunni area does not. That will create some obvious friction.