ISIS had a huge Russian and Central Asian contingent in its heyday. About one out of every five Russian citizens is Muslim, but that population is not evenly distributed either geographically or socioeconomically. In cities, a lot of taxi drivers and hard-luck laborers have names like Magomedov and Ismailov, indicative of Muslim ancestry.... The center of geographic gravity of Islam in Russia is the Northern Caucasus, the site of domestic strife and bloodshed in a series of episodes going back centuries. Some groups around Dagestan and Chechnya have become proficient guerrilla warriors, and Putin perfected his own harsh methods on...