In two very different court proceedings last week, evidence was presented which you may find helpful if you’re still trying to assess President Trump’s rationale for his border wall. Along with bigness and elemental beauty, he credits it with crime-stopping powers: in particular, he says, it will stanch the flow of drugs that feed our country’s raging opioid habit. In his prime-time address to the nation, on January 8th, Trump told Americans that “our southern border is a pipeline for vast quantities of illegal drugs including meth, heroin, cocaine, and fentanyl.” Without a wall, he tweeted on January 11th, “Criminals, Gangs, Human Traffickers, Drugs & so much other big trouble can easily pour in. It can be stopped cold!”

One of the proceedings was the case that the Massachusetts attorney general filed against Purdue Pharma, the company that produced the powerful opioid painkiller OxyContin, along with some of the company’s directors and eight members of the Sackler family, which owns it. For years, the Sacklers, an art-loving, philanthropic clan, have represented themselves as hands-off proprietors, who were unaware of both the dangers of OxyContin and Purdue’s aggressive marketing of it. (In 2007, Purdue and three current and former top executives, none of them Sacklers, pleaded guilty to federal criminal charges of misleading the public about the addiction risks of OxyContin, and paid $634.5 million in fines.) But a court filing released last week disclosed the exulting of one family member, Richard Sackler, that OxyContin would create “a blizzard of prescriptions,” which he predicted would be “dense, deep and white.” It also contained his recommendation, in an e-mail, “to hammer on abusers in every way possible”—on people, in other words, who had become dependent on a drug that was extremely addictive, and, thanks in large part to Purdue’s assurances that it was not, was wildly overprescribed. The case was a reminder than an opioid epidemic that is largely responsible for lowering life expectancy in the United States for the first time since 1980 began quite legally, with a drug manufactured not by a Mexican cartel but by an American pharmaceutical company.

The second proceeding was the trial of Joaquín (El Chapo) Guzmán Loera, the suspected leader of the Sinaloa drug cartel, which has been under way in New York since early November. This week, a former high-ranking associate of Guzmán’s, named Jesús Zambada García, was testifying about the cartel’s business practices. Asked how it typically smuggled drugs into the United States, Garcia explained that Mexican drug traffickers rely mainly on vehicles driven through official border checkpoints. They used illicit tunnels under the border in the early nineteen-nineties, he explained, but, after the tunnels were detected and closed, they turned to hiding narcotics in legitimate-looking shipments of canned goods and the like. Sometimes they transported them by sea, smuggling drugs in fishing boats or dispatching them in submersibles. One stratagem they did not apparently resort to was sneaking across the Mexican border between checkpoints—the gaps that a wall would fill.

This testimony comported with what Daniel Ciccarone, a researcher who heads the Heroin in Transition project at the University of California, San Francisco, had told me. If you’re going to put money into drug interdiction—an endless task, he said, given how nimble and persistent the cartels are in responding to demand—“Where are you going to get the most bang for your buck? Is it going to be stopping a person hiking through the desert to get to Texas? Or is it going to be stopping trucks?” He answered his own question: “The tonnage is coming in through trucks and cars that go through border checkpoints—so put the money there.” (The compromise bill on border security that the Democratic-led House passed on January 3rd does just that, allotting additional funds to “non-intrusive inspection devices” at ports of entry.)

The Drug Enforcement Agency’s 2018 National Drug Threat Assessment underlines the point: the majority of the heroin that gets across the southwest border, which is indeed where most heroin enters the United States, comes in privately owned cars, trucks, and tractor-trailers “entering the United States at legal ports of entry.” (Cocaine predominantly arrives in vehicles at official points of entry as well, according to the D.E.A, though commercial air travel is another route.)

The opioid epidemic appeared in three waves, starting in the nineteen-nineties: the first was of prescription pills, mainly OxyContin; the second was of heroin, which replaced pills for some opioid-addicted people, once new prescribing practices and less abusable formulations rendered pills less available; and the most recent took the form of fentanyl and other synthetics, which are cheaper to produce than heroin and fifty times more potent. Ciccarone and his U.C.S.F. colleague Sarah Mars have conducted research showing that the fentanyl wave is a product more of supply than of demand. Most users don’t particularly want fentanyl—they know the risks of overdosing on a much stronger drug—but it’s what’s available now. And it is often branded and sold in powder form as heroin, or formed into counterfeit pills, making it difficult for users and even street-level sellers to know what they’ve got.

Fentanyl is used for medical purposes, such as pain relief in cancer and epidurals in childbirth, and is manufactured above-board in this country, where it is a classified as a controlled substance. But illicit fentanyl, which is now at the leading edge of overdose deaths in the United States, has a much more complicated provenance. According to the D.E.A., most of it is made in clandestine laboratories in China. Though some is then smuggled in across the southwest border, much of it is mailed or shipped. Last June, at the port of Philadelphia, U.S. Customs and Border Protection agents seized a hundred and ten pounds of fentanyl that was concealed in a shipment of iron oxide from China. (Alerted by a narcotic-sniffing dog, the agents tested the substance with a handheld isotype-analysis tool.) Paul Knierim, the D.E.A.’s deputy chief of operations for the office of global enforcement, testified last September, at a congressional subcommittee hearing titled “Tackling Fentanyl: The China Connection,” that “China is one of the world’s top producers of the precursor chemicals used to manufacture methamphetamine and fentanyl.” When the D.E.A. takes action to ban the import of certain chemicals, “Clandestine chemists can easily continue developing and synthesizing new synthetic opioids that do not appear on any schedule of controlled substances,” Knierim said, sometimes in a matter of weeks. “Sadly, these substances are often first discovered when D.E.A. receives reports from local hospitals and coroners in connection with a spate of overdoses.” He added that, because of its potency, “one kilogram of fentanyl purchased in China” carries the potential of “being lethal for five hundred thousand people.”

The fact that only small amounts of fentanyl are needed to supply a very large population makes detecting and disrupting imports that much harder, Mars told me. This is even more the case with fentanyl substitutes such as carfentanil, a synthetic opioid that is a hundred times more powerful (it’s used as a tranquilizer for elephants and horses) that has begun to turn up in the U.S. heroin supply over the past few years.