Believing he was answering a holy call, Harry Sarfo left his home in the working-class city of Bremen last year and drove for four straight days to reach the territory controlled by the Islamic State in Syria.He barely had time to settle in before members of the Islamic State’s secret service, wearing masks over their faces, came to inform him and his German friend that they no longer wanted Europeans to come to Syria. Where they were really needed was back home, to help carry out the group’s plan of waging terrorism across the globe.The masked man explained that, although the group was well set up in some European countries, it needed more attackers in Germany and Britain, in particular. “They said, ‘Would you mind to go back to Germany, because that’s what we need at the moment,’” Mr. Sarfo recalled. “And they always said they wanted to have something that is occurring in the same time: They want to have loads of attacks at the same time in England and Germany and France.”