Sergio “Pan Duro” seemed intent on teaching a lesson. The former human smuggler, whose nickname translates literally as “hard bread,” had led a group of Mexican and US humanitarian-aid volunteers on a dusk patrol to the summit of a thousand-foot desert mountain. To the northeast, the lights of Agua Prieta, Sonora , and its conjoined border twin, Douglas, Arizona, shone across the valley floor. Around the group, the outlines of prickly pear and cholla cacti, thorned mesquite and ocotillo could be only dimly perceived. But they were felt, as they repeatedly snagged clothes and raked exposed skin. The US walkers hung close together, pulling aside branches for one another, clutching arms when stones rolled underfoot, offering a hand after the inevitable hard backside falls. Full darkness had descended by the time they returned to the flat, and they wandered lost for a few tense minutes in dense, head-high brush, before Pan Duro appeared with a flashlight and guided them back to camp.