Cannabis is often described as the “cash crop” of Mexican cartels because it grows abundantly in the Sierras and requires no processing. But it’s bulkier than cocaine, and smellier, which makes it difficult to conceal. So marijuana tends to cross the border far from official ports of entry. The cartel makes sandbag bridges to ford the Colorado River and sends buggies loaded with weed bouncing over the Imperial Sand Dunes into California. Michael Braun, the former chief of operations for the D.E.A., told me a story about the construction of a high-tech fence along a stretch of border in Arizona. “They erect this fence,” he said, “only to go out there a few days later and discover that these guys have a catapult, and they’re flinging hundred-pound bales of marijuana over to the other side.” He paused and looked at me for a second. “A catapult,” he repeated. “We’ve got the best fence money can buy, and they counter us with a 2,500-year-old technology.