I think Kilcullen argument that the Islamic State is in fact a state based upon the Montevideo Convention is debatable.

Kilcullen wrote,

But consider the definition of a state in international relations, which is generally agreed to require the fulfilment of four criteria: (1) a state must control a territory, (2) that territory must be inhabited by a fixed population, (3) that population must owe allegiance to a government, and (4) that government must be capable of entering into relations with other states
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Does IS control the territory anymore than any occupying military power? Is the population fixed or held captive? Does the population writ large owe allegiance to IS? Point 4 is debatable, I recall reading elsewhere a state must recognized by other states to qualify for state status and the legal rights associated with that status. The Montevideo Convention doesn't require this specifically, but it is rather difficult to enter into relations with another state if that entity is not recognized as a state.

A short, but decent description of this debate can be found at the following link.

https://thenewinternationallaw.wordp...makes-a-state/

Arguments can go round and around about the importance of recognition over fulfilling the Montevideo elements. The question still remains: what is it that makes a State? Articles 3 and 6 of the Montevideo convention make it clear that the recognition of an entity of as a State is not what makes it a State. However, even that convention makes room for recognition as an element in its requirement that the new State be able to enter into international relations. I propose that “Statehood” is the product of a balance between the Montevideo criteria and recognition. The more you have of one (criteria or recognition) the less you need of the other. However, in all cases, you need a little of both to be a State.
If we accept that the Islamic State is an actual state, then one could argue there would be a legal requirement to declare war upon it based upon self-defense and have that war sanctioned by the UN? I really don't think any real state wants to go there. I do agree with Kilcullen that IS is certainly state building, and may in fact achieve a non-debatable state status in the future. It is getting increasingly difficult to recognize Iraq and Syria as a legitimate authorities over their Sunni population.

Yet Kilcullen writes,

Western countries have a clear interest in destroying ISIS, but counter-insurgency should not even be under discussion. This is a straight-up conventional fight against a state-like entity, and the goal should be to utterly annihilate ISIS as a state.
I suspect that implies re-establishing Syrian and Iraqi control over their lost territories? That doesn't sound like a recipe for success.