I believe ISIS to be fundamentally different to the ideological extremist terrorism we have seen before. It does not manifest itself wholly in a physical army which one can join, setting physical borders to take or defend. It does not recruit, in the traditional sense.
It does not want borders, or to say it is stronger than its enemies. ISIS just "is". The ISIS concept of an "Islamic State" is a notional one - not a country but the globe.
It creates a 'Facebook' for a Jihadist cause and people sign up to its message and to its ideologies, through its slick social media and internet message. Long before they make the physical connection those that turn to ISIS are persuaded, groomed. ISIS does not go out to find. Its army finds it.
The pioneers were Al-Qaida, which concept evolved into ISIS, which will evolve into something else. Whatever they become, is very dangerous in my opinion and it continues to grow.
We would be wise not to assume that ISIS will be defeated in the traditional sense. Which means that traditional ways of delaying with terrorism won't work. In my opinion.