Around 7:30 on a Friday night in early November Aremi Balboa Victorio, mayor of a small town in southern Mexico, received a call informing her that 1,500 immigrants from El Salvador were heading toward her. The caller, a neighboring town’s mayor, had seen them pass and thought they’d likely stop for the night in Metapa, a tiny Mexican town near the border of Guatemala. Victorio hopped in her car, drove 30 minutes to the nearest city, and filled it with crates of eggs, sugar, toilet paper, and water-cooler size jugs of water. With a rhinestone-manicured hand, she paid the $300 bill from her own wallet.

During the past three weeks, two large caravans of immigrants from Central America had walked past