In the wake of the 2003 American invasion of Iraq and the chaos that ensued, a new variant of radical Islamism emerged. Beginning as a branch of al-Qaeda in Iraq, this group—now commonly referred to as ISIS (Islamic State of Iraq and Syria)—has opportunistically used the civil war in Syria to conquer large swaths of territory bridging both nations. In 2014, the leader of ISIS, the radical Sunni cleric Abu Bakr al-Baghdadi, declared this land to be a new caliphate, or geographical center for the transnational community of Muslim believers, known as the umma. In this speech, Baghdadi calls on all “true” Muslims to migrate to the caliphate in order to serve as mujahideen (or holy warriors) in a jihad (or violent struggle) against their enemies, which include other Muslims as well as the West. ISIS believes that those who fight and die in the service of their new state will become martyrs and enjoy life everlasting, thus achieving true happiness. The willingness of Baghdadi’s followers to heed this call by engaging in spectacles of mass terror—acts which ISIS regards as a just response to Western imperialism and other global assaults on Islam—has shocked the world. While the geographical scope of ISIS’s self-proclaimed caliphate has now disappeared, the idea remains a powerful one for some radical Islamists, and ISIS is still a formidable terrorist threat.