Let me repeat myself.
Muslims, regardless of whether they are Shia or Sunni, will not take part in an infidel-led war against their own people. Because of this, Iraqis will either support the ISIS cause, or will turn sit this fight out.
The fact that ISIS has been able to prop up a military position just 15 miles from Baghdad, is very telling of the kind of defenses the Iraqi military is dealing with.
This is a holy war, nothing more. Muslims will back Muslims against any western interests. Period.