Someone mentioned the whole "You can't destroy an idea" thing, and along with that goes "One man's terrorist is another man's freedom fighter". So I looked for vaguely modern parallels. The closest I could some up with was Northern Ireland and the IRA.
How did we ultimately move forward in the peace process? We gave in and let them win, for the most part. Give them back their land, stay out of the way, and the terrorists as they've nowt to fight for.
Now, I'm not saying that this is the same at all, and really if we simply left the ME to it then ISIS would end up ruling entire countries with massive military weapons which would be A Very Bad Idea Indeed. At some point though we need to either go in full force (you cannot tell me that the combined might of the US, GB, FRA, GER etc armies cannot destroy a ragtag bunch of terrorists if they *really* put their minds to it) so just pull out and wait until the dust has settled.
And then go and kick some arse.