Firstly, I want to make it clear that I do not agree with the term "Islamic State", as a description of the entity currently running rampant through Syria and Iraq.
I'm reasonably sure that the practices employed by these people only vaguely resemble the most extreme form of the most archaic type of Islam, and the term "State" is generally applied to an internationally recognised, self-governing entity which has been validated and has political and cultural ties with other recognised entities. This entity has none of these attributes. By calling it a "State", we are bestowing a legitimacy which is unwarranted and unearned.
Secondly, I need to make sure that you realise how complex this situation really is, and that there is no simple solution, whether short or long-term. The movements and cultures that have been formed in the regions affected by this political situation have their roots back in the Sixteenth Century, if not earlier, and have been moulded by the ensuing events, including the Ottoman, British and American (Economic) Empires. Something formed over such a long period will not disappear overnight. Ask the Croats and Serbs...
So,
The Terrorist Army (as I shall refer to it), has gained a substantial footing in the middle of Iraq and parts of Syria at this time. It is trying very hard to gain access to oil wells and cities for cash and commodities, and has actually succeeded in places. Where to from here?
Most current affairs buffs will talk knowledgeably about the Kurds holding the North of Iraq, and the push for the terrorists to take the South, with it's many oilfields in order to strengthen their position, and they are right.
What they will not talk about is that if they gain control of these areas...it's going to be a long day for Western Civilization. Here's why.
Any terrorist or insurgency inspired State has by it's very nature, a limited life-span. If such a state is founded upon the forceful personality of a single individual, then upon that persons' demise, things crumble rapidly. Particularly if no stability and line of succession is established beforehand. Witness any one of a hundred such regimes in Africa in the last 60 years. You can only slaughter your way to power for so long, eventually the slaughter comes back at you.
Killing is a very effective form of control, but eventually it loses it's effectiveness. Pol Pot discovered that killing everyone left you with no workers, and finally those that were left realised that it was necessary to get rid of their leader before he killed them all.
If this were the situation in the Middle east, then all we would have to do is wait, because, let's face it, these monsters are killing many thousands. eventually it will have to stop and then it would fall down. BUT...
If, when the killing stops, the leaders of the Terrorist Army can substitute slaughter with another, more subtle form of intimidation, then they are half-way to settling in for the long-haul. They could use religion, such as Iran has used very successfully for the last 35 or so years, keeping the population tightly controlled through fear of Damnation and prison, or they could take a leaf out of the West's handbook.
Saddam Hussein, Ali Basheer, Moamar Gaddaffi, and a half dozen other leaders successfully kept power to themselves through sheer force of arms....and the support of a handful of Western nations that wanted their oil. Should the Terrorist Army gain enough oil wells, they will have large sums of ready cash at their disposal. There will always be a market for oil. When Iraq had an oil embargo placed upon it by the U.S. and its allies, they still managed to sell oil to small, desperate nations at discount prices. If they can hold onto this area long enough, they will start moving toward a form of recognition by simply being important as a supplier of oil. With a government comprising religious leaders, such as the Ayatollahs in Iran, stable lines of succession to power are established, which minimises disruption when power is handed from one leader to another. it works for the Catholic Church too, remember.
Of course, the cash from the oil will allow them to buy arms, if not from the U.S., then Russia, or China, or some third-party nation with a need for oil and little cash reserves.
Should this situation arise, then the fanatical fire that calls itself the Islamic State will not burn until it destroys the foundations that built it, like so many before. Instead it will feed on the oil that it has captured and use it to try and create a conflagration that will engulf the Middle East. Should this occur, it may burn right through to North Africa, including Egypt, and down into failed States like Liberia and Somalia, where there is fuel aplenty for such a fire as this.