> That means it isn't going to calm down, but it also means that its claims to legitimacy based on fulfilment of prophecies about unification of territories are more readily defeated by military means than other jihadists' claims to be defending the Muslim world from infidels.
I agree. The fact that ISIS effectively seeks to become a nation state makes fighting them a lot more straight forward. It's very difficult to fight an insurgency or terrorist group which is not interested in holding territory and blends in with the population. It's pretty straight forward to bomb, lay siege to, or generally bleed a group which needs to have government buildings, infrastructure, police, logistics etc. The fact that their legitimacy is primarily based on continuous expansion is also great, as defeat creates a highly negative feedback loop for them making recruitment more difficult.
A key difference between al Qaeda, which could broadly be defined as trying to "tip the scales eventually toward a Caliphate" whilst battling for certain populist causes and ISIS is that the latter considers itself to be a Caliphate, and is actively pushing for the Islamic equivalent of Armageddon. That means it isn't going to calm down, but it also means that its claims to legitimacy based on fulfilment of prophecies about unification of territories are more readily defeated by military means than other jihadists' claims to be defending the Muslim world from infidels.
Edit: see also this user's point and linked article https://news.ycombinator.com/item?id=10581489