The relationship between the murderous zealots of ISIS and the rest of the Muslim world is too complicated to sum up concisely. It goes without saying that hatred between Sunnis and Shias lies at its heart. They adhere to profoundly different versions of Islam: where radical Sunnis are disgusted by cultic practices or religious art that distract from the teachings of Mohammed, Shias embrace a messianic cult of martyrdom and ritual self-mortification – and claim a line of descent from the Prophet that Sunnis regard as heresy.
This fault line dates back to the early years of Islam and is familiar to anyone who knows the first thing about the religion. But to make sense of the new Iraqi civil war it's also necessary to untangle the relationship between the fanatics of ISIS and the kingdom of Saudi Arabia, notionally an ally of the West. The pampered aristocracy of the House of Saud may not wish to be publicly associated with terrorists, but it is the Saudis who, since the 18th century, have nurtured the ultra-puritan Wahhabi ideology adopted by those same terrorists. Wahhabis or Salafists (the two terms are almost interchangeable) seek religious purity through iconoclasm. In Mecca this takes the form of the Saudis razing every shrine associated with Mohammed, lest they encourage superstition, while simultaneously building hotels modelled on Las Vegas. In Iraq and Syria, non-Sunni places of worship are also razed, but the iconoclasm is accompanied by the slitting of throats.
Many rich Saudis are secretly thrilled by the advance of ISIS, whose atrocities are an extreme manifestation of their own Wahhabi ideology. And they will gloat mightily if ISIS fulfils its ambition of reducing every Shia shrine in Iraq to blood-spattered rubble. As we speak, funds are being transferred from their bank accounts to the organisers of the insurgency, who despise Saudi princelings for their "Western" lifestyles but are more than happy to pocket the cash.
The transactions and alliances surrouding this civil war are sordid and cruel on both sides. But this is in the nature of wars of religion, which throughout history have combined barbarism and hypocrisy. The conflict between Catholics and Protestants in early modern Europe produced scenes that were as awful as those we are witnessing today. We did, however, grow out of it. The Muslim world still hasn't and you can't help wondering if it ever will.