I don’t think it is that simple.
Iraq is more Shiite (Shia). Syria is more Sunni. ISIS is fighting to take over both (and everyone else, eventually)
It is that simple.
Iraq is about 60/40 shia/sunni.
The sunnis support IS.
The “great awakening” and the “surge” are when the sunnis turned on AQ.
Iran is Shia. Saudi Arabia is Sunni
Syria is about 50/50.