The closest analogy to ISIS is probably Sri Lanka’s own LTTE. Not in terms of religious zealotry or gleeful brutality, but in terms of being a terrorist group that controls land. It’s a rare phenomenon that bears comparison.
The LTTE, at one point, controlled 76% of the Northern and Eastern provinces of Sri Lanka. They had a police force, media and controlled check points in and out. They also murdered any opposition and relied on violence to cement power, much like a state does, though being young they had to apply it much more often.
The LTTE was a terrorist organization in that, well, they basically invented modern suicide bombing and a range of innovative terrorist tactics. You could say this was the only way they could defend what they wanted (a mono-ethnic Tamil state) against shittily democratic Sri Lanka, but they were undoubtably a terrorist organization in the modern sense of the word.
ISIS or ISIL or Daesh or whatever is an evolution out of Al Qaeda in Iraq, feeding off the instability there and in Syria. Whereas the LTTE actively created the absence of a state, ISIS just filled a void as states collapsed. American clusterfucking of Iraq and climate-change (drought) induced collapse in Syria created a gap that they just filled.
What they filled it with is something they call the Islamic State, though both labels are debatable. Broadly they suck, but they also provide services and a quasi-monopoly of violence in the territory they control.
What ISIS and the LTTE have in common is that both are/were awful, in that their fundamental premise is wrong. Both groups are about the mono-ness, that is, being one ethnicity or religion.
For modern states that’s basically incompatible with individual rights. ISIS is about everyone being one brand of Islam and the LTTE was about everyone being Tamil. Both have some allowance for minorities (the jizya tax or vaguely being allowed to exist under the LTTE) but the basic foundation of each ‘state’ is discrimination. Which is not cool.
What both the LTTE and ISIS do is create a semi-monopoly of violence in territory they control. That’s the basic function of a state, which is what makes them sorta states. It’s what makes them appealing to the people that live there, the alternative being chaos.
The way to defeat them is to take that semi-monopoly away and replace it with another order (ie, another state). Sadly, in Sri Lanka this was done through violence.
The way to defeat terrorist states like this is to shore up the actual state and let it re-assert a monopoly of violence. In Sri Lanka that meant the Sri Lankan government and military re-asserting territorial control, painfully, and continuing to maintain a military presence in the North and East. Where there are ethnic lines this is problematic. The Sri Lankan military is mostly Sinhala and the previous government was a bit racist, so the situation is flawed at best. Giving the actual state a monopoly of violence, however, is the only way to defeat the terrorists groups trying to control the same.
Regarding ISIS, this would mean shoring up either the Iraqi or Syrian states and helping the assert control. The trouble is that the Iraqi Army is shambolic and corrupt, whereas the Syrian regime is more straight-up evil. They make Mahinda’s government look like Canada.
That, however, is the only way to actually defeat ISIS. There is another option, which I think America should follow, of actually taking over the places they invade and making them states in the American union, with voting rights and all. Current American policy seems more geared towards feeding their military industrial complex, that is, blowing up all the munitions and gear they buy from suppliers, ie, playing with all their toys, at the expense of lives.
At some point just randomly bombing land is not going to do it. To defeat a terrorist state you have to support some other state. Unlike in Sri Lanka, however, there is not a (comparatively) decent state to take control. The choice in Iraq and Syria is to support at best corrupt and at worst despotic and murderous states. To defeat ISIS, however, that may be the only choice.