"All Muslims acknowledge that Muhammad’s earliest conquests were not tidy affairs, and that the laws of war passed down in the Koran and in the narrations of the Prophet’s rule were calibrated to fit a turbulent and violent time. In Haykel’s estimation, the fighters of the Islamic State are authentic throwbacks to early Islam and are faithfully reproducing its norms of war. This behavior includes a number of practices that modern Muslims tend to prefer not to acknowledge as integral to their sacred texts. “Slavery, crucifixion, and beheadings are not something that freakish [jihadists] are cherry-picking from the medieval tradition,” Haykel said. Islamic State fighters “are smack in the middle of the medieval tradition and are bringing it wholesale into the present day.”

The Koran specifies crucifixion as one of the only punishments permitted for enemies of Islam. The tax on Christians finds clear endorsement in the Surah Al-Tawba, the Koran’s ninth chapter, which instructs Muslims to fight Christians and Jews “until they pay the jizya with willing submission, and feel themselves subdued.” The Prophet, whom all Muslims consider exemplary, imposed these rules and owned slaves.

Leaders of the Islamic State have taken emulation of Muhammad as strict duty, and have revived traditions that have been dormant for hundreds of years. “What’s striking about them is not just the literalism, but also the seriousness with which they read these texts,” Haykel said. “There is an assiduous, obsessive seriousness that Muslims don’t normally have.”

http://www.theatlantic.com/features/archive/2015/02/what-isis-really-wants/384980/