Worry is rising over the risk of terrorism at Russia’s 2014 Winter Olympics. Last week’s deadly attack on a hydroelectric station in Russia’s deep south only added to the concern. The number of attacks in the predominantly Muslim North Caucasus was up 57 percent last year, and unlike the Chechen wars of 1994–2001, these killings have been the work of a bewildering array of rebel groups, some motivated by radical Islam but others by separatism or clan warfare.