WHEN Islamic State (IS) declared on June 29th that its leader was the new caliph, no one was more perplexed than his supposed flock of 1.6 billion Muslims. All but the most avid jihadists reject the claim outright. In the case of the 126 distinguished scholars and clerics who recently penned a lengthy, 24-point refutation, the dismissal was carefully argued on grounds of Islamic jurisprudence: a caliph can only be chosen by general consensus; anything else would be fitna, Arabic for strife or sedition.

The yearning for the caliphate, and its propaganda value to IS, are understandable. Schoolbooks paint the rule of the first four caliphs or “successors” to Muhammad as a golden age. Arabs wistfully recall the glory of the caliphates of 8th-century Damascus and 9th-century Baghdad. Modern Islamism, with its leitmotif of grievance against the West, has long harped on the fall of the last titular caliph, in 1924, as a deep wound to the faith. Even the doubting signatories of the letter admit that establishing the caliphate is “an obligation upon the ummah” (or Muslim “nation”).

And yet the institution had been in decline long before Turkish republicans deposed Abdul-Majid II (pictured), the last Ottoman sultan and titular caliph, who ended his years in Paris painting and collecting butterflies. His claim was in any case thin. The Ottomans had acquired the title by war: among booty from their conquest of Egypt in 1516, along with hairs from the prophet’s beard, was the last caliph of the previous, Abbasid line, who surrendered the title in exchange for his freedom. His forebears had fled to Egypt following the Mongol conquest of Baghdad in 1258, and had for centuries been paraded on state occasions to legitimate the rule of Egypt’s Mameluke sultans.

Long before the Mongol onslaught, Abbasid generals and ministers had already assumed more real power than their ostensible masters, several of whom were famed for drunkenness or debauchery. Besides, in the 10th century, rival caliphs in Cordoba and Cairo vied for dominance. Even in the earliest times, clashes between different lines of descent from the prophet often cast doubt on the legitimacy of caliphs. Some insisted the office should not be inherited at all. Instead, as happened with Muhammad’s immediate successors, caliphs should be chosen from among those most qualified. (Such disputes gave rise to the Sunni-Shia divide.)

Supporters of IS’s supposed caliph, who calls himself Abu Bakr al-Baghdadi, say that he is a pious man and a skilled commander, as well as a descendant of the prophet. Since he has established a state, imposed Sharia (Islamic law) and “removed evil”, Muslims must now atone for abandoning the caliphate, and pledge allegiance. Perhaps so, but the Koran is more ambivalent. When God tells the angels he will place a caliph on earth they ask, will he do harm and shed blood? God replies, “Surely I know that which ye know not.”