Voted "other" as it is more complicated than that.
But I might be willing to say that ISIS would not have the numbers today if Saddam was still in power *and* al-Assad had better control over his nation as well *and* other nations in the region also had better control over their respective nations. Like Egypt, Afghanistan, Pakistan, Yemen and a handful of others.
As sad as this is going to sound most in that region seem to be better off under a military and/or religious dictator than otherwise.
There is something about that culture and ideology suggests that western world leaning concepts of freedoms and tolerances are simply incompatible with most of that region. That does not mean I think Saddam, or al-Assad, or leaders of these other nations are peaceful people. Just saying that for this culture when absent of a clear and definitive ruler we have issues with pockets of the culture turning on each other and everyone else with horrific results. Terrorism, Islamic Fundamentalism, whatever you want to call it... all seems to have a direct correlation to region instability. Sometimes our interventionism seems to be a core cause, other times the mix is too complex to blame on any one party.
But in this case for ISIS, we seem to have a group that capitalized on a multiple way civil war in one nation and a weak government and military in another. The latter we are directly responsible for. As much as I am against all of this, leaving a residual force to handle Iraq might have slowed down ISIS being able to flood from Syria into Iraq and mobilize just enough people that have been in Iraq for generations anyway thinking along the lines ISIS demands.
And if you objectively look at the region and when problems come into power, US and European hands tend to be involved. Hard to judge otherwise, we seem to cause a significant number of our own problems. Seems to me it is because we continually go into the region expecting people look at ideology the same way we do, only to find there region by majority does not have the aptitude for such ideology. For instance, democracy means little to a group that looks to a religion where the text itself has a baked in ideology for government authority. And that authority has nothing to do with freedoms or tolerances.