A Franco-Syrian journalist’s investigation links far-left leaders to Islamist networks. A book that has not yet gone on sale is already causing tremors in French politics. It is Les Complices du mal by Franco-Syrian journalist Omar Youssef Souleimane, and will be published in early October. The volume, the result of extensive field research, reveals the links between leaders of La France Insoumise (LFI), the far-left party of Jean-Luc Mélenchon, and Islamist organizations close to the Muslim Brotherhood and their growing influence in French political and social life. The controversy erupted when LFI’s lawyers sent a legal notice to the publisher...