NOGALES, Arizona/Sonora—A fence has cut through the border city of Nogales for decades, dividing the Arizona side of town from its more sprawling Mexican neighbor.

“At one time, it was very friendly,” says Tony Estrada, sheriff of Santa Cruz County in southern Arizona. “The fence was made of chain link. It had gaps and holes.” But a surge of drug-running and illegal immigration during the early 1990s convinced the federal government to strengthen the fence with dark, steel-mesh landing mats used by the U.S. military to improvise airstrips during wartime.

The Border Patrol installed some of these surplus mats edgewise along the border to create a tighter, darker fence. This barrier foiled some aggressive smuggling but failed to take into account “our neighbors across the line,” says Estrada, meaning Mexicans in Nogales, who found it ugly and disrespectful. Members of smuggling cartels learned to cut the mesh mats with acetylene torches.