The military victories of the Islamic State have completely overturned the geopolitical map of the Middle East. Media attention has focused almost exclusively on Islamic State's savage treatment of its enemies and its ability to attract foreign fighters. In 2011, the first effort to build an Islamic State in Iraq was defeated by US and Iraqi forces. The second attempt to establish themselves, beginning in 2014, has been considerably more successful and that success calls for deeper investigation. In order to explain the successes of Islamic State, The Caliphate at War brings together a dispassionate and objective account of the significant innovations in insurgency, ideology, and governance. By focusing their ideology first and foremost on extreme anti-Shia sectarianism - rather than on Western "infidels" - the Islamic State's founders are able to present themselves as the saviors of what they see as the embattled Sunni "nation" in Iraq. Its success in taking and holding major cities, including Mosul, demonstrates its innovative tactics and skill at exploiting tribal and sectarian rivalries. By going beyond the often starkly unpleasant current affairs of the Islamic State, The Caliphate at War undertakes an essential investigation into the successes of the group, to better understand how the movement is surviving, thriving, and reshaping the Middle East.