SAHARA DESERT, Niger — A group of light armored vehicles skated over the moonscape of the Sahara, part of one of the largest detachments the French military has deployed here since colonial times. Its mission is growing ever more urgent: to cut smuggling routes used by jihadists who have turned this inhospitable terrain into a sprawling security challenge for African and international forces alike. Many of the extremist groups are affiliates of Al Qaeda, which has had roots in North Africa since the 1990s. With the recent introduction of Islamic State franchises, the jihadist push has been marked by increasing,...