Credit for this mess goes to the United States, due to entering in first place back in 2003.
We should have mediated a population exchange were the Shia went to Syria and the Sunni went to Iraq. That would have ended ISIS before it started.
You really don't...get the demographic geography of the Middle East, do you?
While Syria is mostly Sunni and Iraq mostly Shia, their respective minorities hold the critical, nation controlling parts of the countries. The Alawites control the coasts which controls food, meaning they can starve the Sunni majority if they retaliate. The Sunnis in Iraq control the desert and, just as the Mamlukes and Ottomans did before them, can overrun southern Iraq.
While majoritarianism makes sense at first blush, looking deeper reveals the truth about Iraq and Syria.