Just as Nigeria gets to grips with militants who brought the nation’s oil industry to its knees a few years ago, another group of longstanding foes are slowly making a comeback: thieves. Saboteurs including thieves caused an 80% increase in the number of spills in 2018, Royal Dutch Shell Plc, the largest international producer in the West African country, said in a report last month. By contrast, there have been no militant-related halts to operations since 2016. The disruptions underscore how hard it will be for Nigeria to fully rid itself of security challenges that have plagued the natio...