Russia will go in and carpet-bomb the crap out of the area figuring it’s better to kill 1000 civilians if it gets them 1 ISIS fighter. We’ll see how that goes over with the locals.
Machiavelli said "Do not do your enemy a small injury", by which he meant do not use just enough force to piss him off, while not impairing his willingness and ability to retaliate.
In WW2 we devastated Germany and Japan. Yet we don't see Japanese and Germans demonstrating in the streets in 1946 about any desire for revenge. Nobody there wanted to give us a reason to come back and finish the job.