By not effectively replacing the leader of a country, it leads to a power vacuum, looking for someone to take that power. ISIS grew to fill that void. If you displace them, someone else will come along and take that power, potentially even worse than ISIS
But lets be honest, go back to the Treaty of Sèvres and subsequent assigning of territory to European countries and how it was done without regard to the religious and ethnic traits of those living there. As in, European Colonialism started this mess as it did in Africa. This did not happen in the last twenty years, this is generation after generation finally reaching a point to where they could act.
If you keep arming all sides and leave it is to be expected.
They sold it to the U.N., the world bought it and now look what the US has to show for it, smoking holes in the ground...
http://content.time.com/time/magazine/article/0,9171,998512,...
Anyhow if we sort ISIS who will they blame for restricting freedoms at a time when rampant inequality is causing domestic unrest?