David Kilcullen is one of the world's foremost experts on IS and he's written a new Quarterly Essay called Blood Year: Terror and the Islamic State. It's out this week. He's with me now from Washington. David Kilcullen, let's start with the fall of Ramadi on the weekend because it goes to the heart of something that you've written about in your Quarterly Essay, which is the strategy. How is it possible that you've got the might of the US and its allies on one side, versus a bunch of murderous Islamist zealots on the other who control a patchwork quilt of land and it's the zealots who win a city like Ramadi?