As Graeme Wood explained in his recent cover story for The Atlantic, ISIS holds “a sincere, carefully considered commitment to returning civilization to a seventh-century legal environment, and ultimately to bringing about the apocalypse.” His use of the term 'apocalypse' is not hyperbole. ISIS believes their army is the vanguard of the End Times, ushering in the long-prophesied redeemer of Islam, a Messianic figure known as the Mahdi, who will finally appear to rid the world of evil and make the entire planet Muslim.
The prophecy of the Mahdi is believed by hundreds of millions of Muslims. Wood’s article in The Atlantic finally brought this to the attention of the mainstream press, but an American scholar named TimothyFurnish has been warning about the dangers of “Mahdism” for decades. He says belief in the Mahdi has a long history within Islam:
“Apocalyptic traditions and movements, led by a Muslim claiming to be the End Time Mahdi ("rightly-guided one"), are not new with ISIS or Jabhat al-Nusrah or any of the other modern groups proclaiming belief in such. They go back to the early days of Islam, and are intrinsically connected to the more general Muslim practice of jihad, or holy war against 'infidels.'"
Furnish holds a PhD in Islamic, World and African history, is a former US Army Arabic linguist and officer, and currently works as an author, Islamic World analyst and consultant to the US government and military. His informative website, MahdiWatch.org, was a great resource for me in writing The Assassin Lotus, which involves a cult of Mahdist assassins seeking to trigger the "Final Battle." Here is Furnish explaining the religious basis for that fight:
“A very important point which no one in the analytical, and few in the journalistic, community wants to admit (hence State's Marie Harf adducing phantoms such as poverty-driven jihad): the ISIS Caliph and his minions refer to the United States of America as 'defender of the cross.' Not 'proponent of Ayn Rand,' 'guardian of the Enlightenment' or 'warden of Jeffersonian democracy.' Caliph al-Baghdadi and his decapitating/immolating rank-and-file forthrightly (if inconveniently) spell out exactly why they hate us: because, in their eyes, we are a Christian nation. There are those who will dismiss this as a mere progagandistic trope. But they would be wrong to do so. IS, along with Boko Haram and al-Qa`ida and Jabhat al-Nusra and the Taliban (to name only a few of the Islamic legions), as well as the non-terrorist but Muslim fundamentalist movements such as Wahhabism, Deobandism and Salafism, all view the world through a simplistic but legitimately Islamic lens of Dar al-Islam v. Dar al-harb: the 'house of Islam' v. the 'house of war.' And for 14 centuries the vanguard of the latter has been Christendom. Some decry pointing this out as crass 'Crusaderism.'
But as that combat veteran J.R.R. Tolkien pointed out--via his main female protagonist, Éowyn, 'it needs but one foe to breed a war, not two'--and when that enemy declares its war on us in religious terms, why should we pretend otherwise?”