Stephen Sackur speaks to writer and publisher Jürgen Todenhöfer, who embarked on one of the most hazardous journeys imaginable for a western journalist. Last December, the 74-year-old German spent 10 days inside the territory controlled by the so-called Islamic State movement. He was taken to the group's base in Raqqa, Syria, and then to their most highly prized asset in Iraq - the northern city of Mosul. He emerged unscathed with a remarkable story. What motivates the jihadist fighters?