Former federal agent Wes Tabor says his phone has been lighting up with calls from police departments around the U.S. for advice on how to combat the growing threat from the Venezuelan gang Tren de Aragua. Tabor was in charge of the U.S. Drug Enforcement Administration’s office in the Venezuelan capital of Caracas in 2012 when the gang was still new and when Tabor had barely heard of it. Venezuela had long been a major transit zone for cocaine smuggled by Colombian guerrillas, with a...