Graeme Wood has produced a masterful dissection of how the Islamic State's religious devotion drives its strategy and tactics. It's a long piece, but I highly recommend it to anyone who is interested in understanding what ISIS wants and how it views itself in the world.
Here is one small passage that describes how ISIS is forbidden from engaging diplomatically with anyone:
Choudary's colleague Abu Baraa explained that Islamic law permits only temporary peace treaties, lasting no longer than a decade. Similarly, accepting any border is anathema, as stated by the Prophet and echoed in the Islamic State's propaganda videos. If the caliph consents to a longer-term peace or permanent border, he will be in error. Temporary peace treaties are renewable, but may not be applied to all enemies at once: the caliph must wage jihad at least once a year. He may not rest, or he will fall into a state of sin.
One comparison to the Islamic State is the Khmer Rouge, which killed about a third of the population of Cambodia. But the Khmer Rouge occupied Cambodia's seat at the United Nations. "This is not permitted," Abu Baraa said. "To send an ambassador to the UN is to recognize an authority other than God's." This form of diplomacy is shirk, or polytheism, he argued, and would be immediate cause to hereticize and replace Baghdadi. Even to hasten the arrival of a caliphate by democratic means--for example by voting for political candidates who favor a caliphate--is shirk.
There's a lot more, and a lot to think about.