ISIS, the Islamic State of Iraq and Syria, claims to have set up a caliphate and wants to be referred to as "the Islamic State". Fawaz Gerges, chair of Contemporary Middle Eastern Studies and professor of International Relations at the London School of Economics and author of  The New Middle East: Protest and Revolution in the Arab World  (Cambridge University Press, 2013) talks about the history of the caliphate - both imagined and real - and why ISIS is trying to establish one (and if they've succeeded).