On June 29, 2014—or the first of Ramadan, 1435, for those who prefer the Islamic calendar to the Gregorian—the leaders of the Islamic State of Iraq and Sham (ISIS) publicly uttered for the first time a word that means little to the average Westerner, but everything to some pious Muslims. The word is “caliph.” ISIS’s proclamation that day formally hacked the last two letters from its acronym (it’s now just “The Islamic State”) and declared Abu Bakr al Baghdadi, born Ibrahim ibn Awwad ibn Ibrahim ibn Ali ibn Muhammad al-Badri al-Samarrai, the Caliph of all Muslims and the Prince of the Believers. For Muslims of a certain hyper-antiquarian inclination, these titles are not mere nomenclature. ISIS’s meticulous use of language, and its almost pedantic adherence to its own interpretation of Islamic law, have made it a strange enemy, fierce and unyielding but also scholarly and predictable. The Islamic State obsesses over words like “caliph” (Arabic: khalifa) and “caliphate” (khilafa), and news reports and social media from within ISIS have depicted frenzied chants of “The Caliphate is established!” The entire self-image and propaganda narrative of the Islamic State is based on emulating the early leaders of Islam, in particular the Prophet Muhammad and the four “rightly guided caliphs” who led Muslims from Muhammad’s death in 632 until 661. Within the lifetimes of these caliphs, the realm of Islam spread like spilled ink to the farthest corners of modern-day Iran and coastal Libya, despite small and humble origins.

Muslims consider that period a golden age and some, called Salafis, believe the military and political practices of its statesmen and warriors—barbaric by today’s standards but acceptable at the time—deserve to be revived. Hence ISIS’s taste for beheadings, stonings, crucifixions, slavery, and dhimmitude, the practice of taxing those who refuse to convert to Islam.

Other Muslims have romanticized the time of the early caliphs—but by occupying a large area and ruling it for more than a year, the Islamic State can claim to be their heirs more plausibly than any recent jihadist movement. It has created a blood-soaked paradise that groups like Al Qaeda contemplated only as a distant daydream.

“There is a mystical belief that, if you just establish the caliphate in the right way, Muslims will come to you and everything will fall into place,” says Fred Donner, a historian of early Islam at the University of Chicago. And it is precisely this promise of inexorable, righteous expansion that has drawn recruits from all over the globe—not just nearby, war-ravaged nations, but England and Australia and France, too. Together, they have formed the most monstrous squad of historical reenactors of all time.

The word khalifa means “successor” (to Muhammad), and as such, a rightful caliph can demand the allegiance of all Muslims. But historically, an applicant for the job has had to fulfill a few conditions. He (always he) must be Muslim, fully grown, devout, sane, and physically whole. Because he is theoretically meant to lead Muslims in battle, missing limbs or a sickly disposition will automatically disqualify him. He must also hail from the Quraysh tribe of the Arabian peninsula, a requirement that turns out to matter a great deal in the case of the current caliph.