I said it a few times before, but the Sunni wing of the Iraqi army prefers to either help ISIS or stay neutral. Most officers are from the Saddam regime and they hate the current regime for their ties with the west and their religion(Shia). Binnie that's why Mosul and a handful of other places were handed down to ISIS together with a lot of supplies and money without even fighting.

And as for the ISIS fighters Faithful is right. I just want to add that most of the younger people are brainwashed by promises of heaven and martyrdom. A normal soldier would try to survive and fight on, while these guys don't give a damn about dying. The idea of martyrdom is one of the biggest reasons for them to fight without thinking about consequences.