I think that withdrawal was a mistake. The method of what we did over there may have been flawed, but once committed, we should have stayed.
Any of the folks over there that put their necks on the line to help us (and by extension, their country) are dead, dying or hiding.
Same as Vietnam. Maybe we shouldn't have been over there, or maybe we should have done it differently. But by interfering and then leaving, we doomed our allies in-country to death.
If we had never gone, they may have not risked life and limb for a futile (without assistance while the opposing side has funding and equipment from USSR (Vietnam) / terrorist finances (Middle East).
You don't go to war and quit. That's a good way to lose the advantage and cost more blood on both sides. Imagine if we quit and tried to contain Germany or Japan in WWII. Yeah not the same situation but still.
This (radical, militarized Islam in this case, but anything causing no holds barred terrorism will fit the bill) is a threat that cannot be ignored, because it will not ignore you.
Personally, I think we should have offered aid to the Kurds and any other peaceful folks. Create a cordon of fire around them.
But now what to do with terrorism sponsors? You can't level their entire country that's barbaric. You can't just decapitate the leadership and hope for the best result that's inhumane if a despot comes to power and slaughters everyone who dares oppose him. Nation building makes it easy for radicals to paint you as foreign invaders.
Maybe we are too soft - handed. Maybe we should take a more Ronan approach.
But give them a chance to win back their independence if there is no major trouble from their sector.
That was a wild idea.
But anyways, doing nothing will not help. What gives you the idea that these people will stop if left alone? That kind of appeasement has been tried before. It never works.
Surgical strikes and Spec ops only work if your Intel is up to snuff.
If it was, we would not be in this mess, durr.

because if we knew ISIL was brewing, I'm sure we would have stayed.