"I honestly can only see this ending with full blown war between the west and the future ISIS controlled Middle East territories. I apologise for my unsophisticated conclusion,..."
It is fairly clear that ISIS as is, never mind larger, means that their extreme Islamist ideology will not just stay within their new Caliphate - so there will be a constant threat to everyone that could strike at any time.
Talk of war is not unsophisticated as we've gone into countries for far less (lol), but EVEN if we had the appetite for it, the fact is ISIS's Caliphate straddles Iraq and Syria; the former does not want western troops in again, the latter (Assad of Syria) being propped up by western boots on ground is hardly better for the people of Syria once we leave.
Similar to Afghanistan we could say that we should not let them have a base from which to attack, but I suspect that like a Hydra, they now have organised tentacles in other States.
Conclusion; we can only step up what we are currently doing and unofficially get the Special Forces of several countries taking out various ISIS targets within their 'homeland' and ANYWHERE outside. IMO