Muslims again are not a cultural group. Its a religion. Italians and germans were cultures (other nationalities). Why do people constantly mix the 2 up?
Why do people constantly insist on placing western world concepts such as 'nationality' and 'ethnicity' on the rest of the world? Especially on places like the Middle East where they do not strictly apply?
Why do you think Iraqi forces fled when ISIS approached Mosul? The Iraqi army had no affiliation with the Iraqi state beyond their loyalty to Saddam Hussein. Shia and Kurdish soldiers had no reason to defend a Sunni city.