The Marshall Project is a nonprofit newsroom covering the U.S. criminal justice system. Sign up for our newsletters to receive all of our stories and analysis. Sinaloa, Mexico — Not long ago I met a small-time rancher here in the birthplace of Mexican drug trafficking. I asked him if he ever got to the United States. He said he had been deported several years ago and couldn’t return. He was caught smuggling black-tar heroin in his shoes at the Tijuana-San Diego border crossing, he said. Wasn’t much — not quite a kilo. He did this to raise the money to buy a couple of cows, or a tractor. In Sinaloa, he said, cobblers do a thriving side business cutting compartments in the soles and heels of shoes and filling them with heroin. There’s a market for this work because so many farmers and ranchers — conservative folks, mostly — subsidize their small-time agriculture with drug money. Almost every other farmer for miles around did the same, he said. He had smuggled drugs 50 times before he was caught. He spent two years in federal prison and then was deported. As he spoke, I thought about the wall Donald Trump wants to build at the Mexican border. Today we struggle with a national epidemic of opioid and opiate addiction, first to narcotic painkillers overprescribed by doctors and now to heroin. As I’ve toured America talking about our opiate scourge, I’ve encountered many people who believe a wall will stop heroin from coming north. Walls, in fact, have been shown to stop people. Illegal crossing has all but ceased in Tijuana because of two walls, including one that starts in the Pacific Ocean and runs for more than 14 miles before hitting a mountain. But walls have not stopped drugs, especially heroin. It is the easiest drug to traffic in small batches across a border because it is so easily condensed — and easy to cut later. The rancher from Sinaloa told me he put a little more than a pound and a half of heroin in those shoes, clearing as much as $12,000 in a single trip to the States. You could never fit enough cocaine, meth or marijuana into a shoe to make it worth the risk. “They’re too voluminous,” he told me. Larger Mexican drug operations, of course, ship bigger quantities hidden in trucks, particularly for long-distance hauls to the East Coast. The United States cannot check every one of the millions of trucks that cross north every year. But many ordinary people traffic small amounts of heroin “a la hormiga” — ant-like. They need the money — for things as mundane as cows or tractors, to finish a house, or simply to spend like kings for a while. A lot of heroin trafficking happens a kilo or two at a time. Other ranchers told me they used hollow hammers, toothpaste tubes, the thick of a woman’s hair, backpacks or the carburetor of a truck. No wall stops that kind of trafficking. It probably won’t stop trafficking in fentanyl, either, the synthetic opioid vastly more potent than heroin. American demand for dope is greater than ever today. On our streets heroin from Mexico remains potent, prevalent and cheap — signs of how much is being trafficked, a lot of it, I suspect, through walled border zones. I am sympathetic to those who view Mexico as a frustrating partner. I lived there for 10 years and have visited it and written about it since then. Necessary changes — in education, environmental enforcement, municipal governance and, of course, law enforcement — happen at a glacial pace, while the political elites bicker over minutiae. I feel the bitterness of parents who have lost a child to heroin, knowing that almost all of what killed their child comes to us from, or through, Mexico. (I suspect they feel like the many Mexican parents whose children have been killed with guns bought easily in the United States and trafficked south.) I understand how those parents could see a solution in a wall. I might even agree if I thought a wall could stop the flow of the most easily trafficked drug. That rancher leads me to believe otherwise. What a wall could stop, however, is the one thing that would curtail the flow of heroin in the global age: a deep, sustained, frank relationship, and cooperation, between the two countries. Mexico has displayed uncommon cooperation with American law enforcement authorities in the last few years – sharing information and extraditing major narcos, of whom Joaquín Guzmán, known as El Chapo, was just the latest. That collaboration is precisely what a wall will corrode. This is not to say we should do nothing. Mexico’s elite needs American pressure to change. Their failures mangle the lives of the hardest-working Mexicans, and that affects us. But President Trump’s proposed wall has provoked the exact opposite result: It has unified Mexico’s political classes, while providing the country one more distraction from the crucial changes needed. Of course, Mexico’s political class could surprise everyone. Its members could use this as a moment to honestly address the reasons Mexico is a country people have been willing to risk death to leave. I hope they do. Historically, though, they’ve retreated into noncooperation. Meanwhile, folks like the Sinaloan rancher will find ways through, because we now thirst for their dope and they need the last room on a house, or a cow, or a tractor. Sam Quinones is the author of “Dreamland: The True Tale of America’s Opiate Epidemic.” This article was produced in collaboration with The New York Times.