Since announcing its arrival as a global force in June 2014 with the declaration of a caliphate on territory captured in Iraq and Syria, the jihadist group Islamic State has shocked the world with its brutality.
Its seemingly sudden prominence has led to much speculation about the group's origins: how do we account for forces and events that paved the way for the emergence of Islamic State? How far back in history does one have to go to find the roots of the so-called Islamic State (IS)?
To the oil shock of 1973-74, when Persian Gulf oil producers used the huge surplus of dollars flowing into their coffers to finance the spread of their severe interpretation of Islam?
To the end of the first world war, when the victorious entente powers sparked resentment throughout the Arab world by drawing artificial national borders we hear so much about today? How about 632 AD, the date of the death of the prophet Muhammad, when the early Islamic community split on who should succeed him as its leader - a breach that led to the Sunni-Shi'i divide that IS exploits for its own ends?
The possibilities seem endless and would make for an entertaining variation on the Six Degrees of Kevin Bacon parlour game (which suggests any two people on earth are six or fewer acquaintance links apart) were the subject not so macabre.
But to look at any and all historical phenomena through a simple string of causes and effects is to ignore the almost infinite number of possible effects that might follow from any one purported cause.
It also opens the door to one of the most pernicious logical fallacies historians might commit: post hoc, ergo propter hoc (after this, therefore because of this). So rather than tracing the rise of IS to one or more events in the past, I suggest we take a different tack.
IS is an instance of a phenomenon that recurs in most religions, and certainly in all monotheistic religions. Every so often militant strains emerge, flourish temporarily, then vanish. They are then replaced by another militant strain whose own beginning is linked to a predecessor by nothing more profound than drawing from the same cultural pool as its predecessor.
In the seventh century, there were the Kharijites (the first sect of Islam), a starkly puritanical group that assassinated two of the early caliphs. Like IS, the Kharajites thought they knew best what and who were truly Islamic, and what and who were not.
In the 18th century, there were the followers of Muhammad ibn 'Abd al-Wahhab, a central Arabian preacher whose followers included Muhammad ibn Saud, the founder of the Saudi dynasty. Believing that the worship of saints and the construction of mausoleums were impious acts, ibn Saud's army destroyed sites holy to both Sunnis and Shi'is in Arabia and present-day Iraq, much as IS targets sites from antiquity today.
During the 19th century, Muhammad Ahmad, a member of a religious order in what is now Sudan, proclaimed himself mahdi (redeemer of the Islamic faith), just as Abu Bakr al-Baghdadi, inventor and leader of IS, recently proclaimed himself caliph (leader of the Islamic faith) - a more prosaic position. Ahmad's army overran Khartoum, where it massacred a British-led garrison and beheaded its commander.
Between Muhammad Ahmad and al-Baghdadi there were many, many others.
While tempting, it would be a mistake to believe that each militant group "gave rise to" the next (although later militants have sometimes drawn from or been inspired by their predecessors). That would be the equivalent of saying that the ancient Zealots (a Jewish sect that fought the Romans) gave rise to militant Israeli settlers on the West Bank, or that medieval Crusaders gave rise to abortion-clinic bombers.
Capturing the imagination of followers
From time to time (it's impossible to predict when), some figure emerges in each tradition who puts his own spin on that tradition. To be successful, that spin must capture the imagination of some of that tradition's adherents, who then try to put it into practice.
Some spins, such as that of contemporary Saudi Arabia's Wahhabis, have sticking power. This is not because they are somehow "truer" than others, but because those who advocate for them are better able to mobilise resources - a core group of committed followers, for instance, military capabilities, or outside support - than others. Most do not.
The first is khilafa (caliphate). Al-Baghdadi believes that Islam requires a caliphate - governance that's in accordance with Islamic law over territory that's under the authority of a caliph (a righteous and knowledgeable descendant of the prophet).
When his forces took over Mosul in the summer of 2014, al-Baghdadi proclaimed himself caliph and burnished his credentials for the job by changing his name to Caliph Ibrahim al-Quraishi al-Hashimi. The last two names signify he's a member of the tribe of Muhammad and a descendant of the prophet.
The second idea al-Baghdadi brought into the mix is takfir - the act of pronouncing Muslims who disagree with IS's strict interpretation of Islamic law to be apostates, which makes them punishable by death. This is the reason for IS's murderous rampages against Shi'is; rampages that even al-Qaeda central finds counter-productive, if not repugnant.
Resurrecting the concept of takfir was the idea of Abu Musab al-Zarqawi, founder of al-Qaeda in Iraq. His strategy was to use the concept to tighten communal ties among Iraq's Sunnis by mobilising them against its Shi'is, thus making post-American-invasion Iraq ungovernable.
Al-Baghdadi has gone one step further, finding the concept useful in his effort to purify the territory of the caliphate which, he believes, will soon stretch across the Islamic world.
Finally, there is hijra, the migration of Muslims from dar al-harb (the abode of war, that is, non-Muslim majority countries) to dar al-Islam (the abode of Islam). Just as Muhammad and his early companions migrated from Mecca to Medina, where they established the first permanent Islamic community.
IS wants a great incoming of Muslims into the caliphate, both because it needs skilled administrators and fighters and because it considers emigration from "non-Muslim territory" to "Muslim territory" a religious obligation.
According to some commentators, al-Baghdadi brought a fourth idea to the table: an apocalyptic vision. They base this on the name of IS's glossy magazine, Dabiq (the site in northern Syria where, Islamic tradition has it, the Battle of Armageddon will take place), articles in the magazine and propaganda videos.
It's not too much of a stretch to attribute an apocalyptic vision to IS - after all, just as every monotheism is prone to militant strains, all are prone to apocalyptic visions as well. Nevertheless, I remain unconvinced that the concept represents a significant part of IS's worldview.
Whatever the future may hold, IS, like some apocalyptic Christian groups, has proved itself so tactically and strategically adept that it has obviously kicked any "end of days" can well down the road (roughly the same distance al-Qaeda kicked the re-establishment of the caliphate can).
Further, much of the IS leadership consists of hard-headed former Iraqi Ba'th military officers who, if they think about an apocalypse at all, probably treat it much as Hitler's generals treated the purported musings of Nazi true believers - with a roll of their eyes.
Foregrounding IS's apocalyptic worldview enables us to disparage the group as irrational and even medieval - a dangerous thing to do. If the recent past has demonstrated one thing, it's that IS thrives when its adversaries underestimate it.