Aerial bombings that destabilise regimes and tempt the young into war; utopian fantasies of a pure society reborn from the ashes; and undergirding it all, psychopathic violence. It's the story of Islamic State, but have we seen this movie before, and what is its ending?

Elliot Brennan, research fellow with the Institute for Security and Development Policy in Sweden and the Pacific Forum Center for Strategic and International Studies in the U.S., has been drawing some alarming parallels between ISIS and the Khmer Rouge, which destroyed Cambodia in the 1970s.