... For years, MS-13’s impact on the U.S. was local—confined to specific neighborhoods and cities scattered across the country as the gang used violence to secure and hold turf. Then, federal officials tracked an alarming development. As MS-13’s influence grew, so did its ambition to leverage its network of local franchises into a cohesive, national brand. That would vault MS-13 into territory once occupied by the Mafia, and now held by Mexican drug cartels. A series of trials that wrapped up this summer in Boston shows how MS-13 is pushing to make that leap by streamlining its management structure and...