The military victories of ISIS have overturned the geopolitical map of the Middle East. Media attention has focused on ISIS' savage treatment of its enemies and its ability to attract foreign fighters. However, in order to explain its success, a dispassionate account of its innovations in insurgency, ideology and governance is needed. The first effort to build an 'Islamic state' in Iraq was defeated by US and Iraqi forces in 2011. That the second attempt, dating from 2014, has been more successful calls for explanation. Hashim argues that by focusing their ideology first and foremost on extreme anti-Shia sectarianism -- rather than on Western 'infidels' -- ISIS' founders were able to present themselves as the saviours of what they saw as the embattled Sunni 'nation' in Iraq. This enabled them to win the support of Sunni communities. Moreover, ISIS' stunning ability to take major cities is a result of its innovative tactics. It sows terror in advance of its attacks by using targeted assassinations to kill key city leaders, and its decentralised regional command structure facilitates an unusual degree of coordination between small assault units. Meanwhile, it is making a serious effort to engage in state-building and population control.