BRUSSELS—One morning in November 2015, Ahmed Khaddine, then 25, was in his apartment in central Brussels, typing away on his computer at his desk, when the front door flew open. Before he really knew what was happening, two policemen burst in, grabbed him, pushed his face down onto the wooden floor and handcuffed him before taking him to the police station. For Ahmed, a son of Moroccan immigrants who was born and raised in Brussels, the arrest had been a long time coming. Many years earlier, during his final years of high school, he had begun attending a local mosque […]