For most of the last century, American foreign policy has been dominated by concerns far from our shores: the Axis powers, the Soviet Union, Southeast Asia, the Middle East, and, more recently, China. But the second Trump White House will have to contend with looming national security problems that are much closer to home. For the first time in decades, U.S.-Mexico relations seem likely to take a front seat — and for good reason. A longtime friend, Mexico also presents a growing array of security and economic challenges to the United States. The Trump administration seems to have a keen appreciation of this fact. Indeed, one feature of the initial slate of announced national security appointees is their focus on Latin America. As Ryan Berg, the director of the Americas Program at the Center for...