How the world is dealing with the militant threat

As a teenager, Jejoen Bontinck enjoyed a brief moment of celebrity in his native Belgium, dancing his way to fame in a televised talent competition. Shortly after, he converted to Islam. By the time he was 18, he had left for Syria.

Mr. Bontinck was indoctrinated and recruited by a group called Sharia4Belgium, prosecutors say. The group allegedly helped funnel dozens of young Belgians into Syria and Iraq. In the process, it turned the small European country into an outsized source of potential jihadis.