My opinion is this is a mess, created by Bush, which Obama is failing at cleaning up. Other possibilities I'm reasonably certain could have been are:
1. If Saddam was still in power there would have been no power vacuum for ISIS to fill and any rebellion would have been put down brutally.
2. If we had never left Iraq our soldiers would have kept getting killed by terrorist, some of whom probably now work for ISIS. Its possible we'd have even been facing another major insurgency because it would likely be spilling across the border from Syria.
3 or 2b. Alternatively, we'd be fighting in Syria in addition to Iraq to remove Assad and stop terrorist groups from growing if our President was more "action oriented" than Obama.
So I conclude removing the dictator and trying to institute a democracy in Iraq was the original bad idea that got us into this situation. Also, instituting democracy was mission creep. We went there originally for the WMDs which we didn't find and likely didn't exist in the first place. Mission failure.
I don't like any of our options right now. We commit more troops lives and waste more money we could better not spend or do nothing and watch ISIS slaughter people and grow in strength.