Earlier quoted context omitted.
IS grew out of the vacuum of a civil war in Syria and is predicated on creating a Caliphate. They cannot physically grow if they are not able to gain ground. Their recruitment message is likely to be significantly weakened if they lose territory and battles. Unlike AQ or even the Taliban of recent years they are not able to avoid pitched battles, they can't melt into the background - they would have to stand and figh…
They can definitely hide in the population. Rakka still have 200,000 people living there. I have no idea which part of that population are hardcore isis member : 40-50k maximum maybe. Assuming you could take Rakka, you can't just execute or deport every one with a weird accent. Mosul is even bigger. How many ground troop would it take to occupy all these cities ? This would be a repeat of the second Irak war, or Afgh…
I'm not saying it is good, but is it better than a pretty shitty situation. Do the other groups regularly crucify civilians and hurl homosexuals off buildings? If we acknowledge we can't impose a new government can we create a new normal whereby we smack groups which get out of line but otherwise stay out of the way. It's actually fairly similar to the old Roman system on the outskirts, and the same approach the British initially adopted in the early days of Empire. It's what the Americans sort of did in Afghanistan before the invasion of Iraq effectively dragged them into a civil war.