Lawrence Wright is the author of The Looming Tower, a history of al-Qaida from its inception through 9/11. Wright's book documents the ideological roots of Osama bin Laden's group in addition to its development as a perpetrator of mass violence. As such, he's well-versed on jihadist belief systems, and he writes at The New Yorker's site today about the ISIS rebels' particular breed of apocalypticism.
Last week in Slate, Daniel Bynum speculated about what might happen if ISIS were to establish an Islamic state in the Iraq-Syria area (short answer: it would not work); Wright's piece is about what ISIS would like to happen, in their ideal world. Their ideas on this front are derived from late "al-Qaida in Iraq" founder Abu Musab al-Zarqawi's thoughts on creating "a vast war inside Islam" that would ultimately eradicate all Shiites:
...for Zarqawi and his network, savagery—particularly when directed at other Muslims—was the whole point. The ideal of this movement, as its theorists saw it, was the establishment of a caliphate that would lead to the purification of the Muslim world. The Islamist strategist Abu Bakr Naji offered a revealing outline of Zarqawi’s method in his 2004 book, “The Management of Savagery.”
Naji proposed a campaign of constant harassment of Muslim states that exhausted the states’ will to resist. He suggested concentrating on tourist sites and economic centers. Violent attacks would create a network of “regions of savagery,” which would multiply as the forces of the state wither away, and cause people to submit to the will of the invading Islamist force. Naji believed that a broad civil war within Islam would lead to a fundamentalist Sunni caliphate.
The break between ISIS and al-Qaida over the former's brutality is well-known; Wright's piece puts it in context of the two groups' views of history. Al-Qaida was fighting the West; ISIS is fighting Shiites, and a vengeance that goes back more than a thousand years—and seeks to replace all secular states with a unified religious rule—apparently requires even more gruesome methods.