There is perhaps no stretch of American land as politicized as the U.S.-Mexico border—and perhaps no place as surveilled.
Physical reminders of that fact are everywhere. There are towering fences and walls. Border agents stand guard, patrolling the boundary in trucks. But border security is becoming increasingly stealthy and high-tech as the government erects a "virtual wall"—a fortification not made of steel and concrete, but drones, surveillance towers, and artificial intelligence.
