As President Trump demands federal funding for a border wall he promised Mexico would pay for, he has leaned heavily on the argument that the wall is the only way to stop illegal drugs from coming across the Southwest border.

“The Wall is a very important tool in stopping drugs from pouring into our country and poisoning our youth (and many others)!” the president tweeted this week. “If the wall is not built, which it will be, the drug situation will NEVER be fixed the way it should be!”

But experts on the drug trade say a border wall, even one as big and beautiful as Trump promised, would be near-impotent in stemming the supply of illegal drugs.

The primary reason? Drugs that flow across the Southwest border, like heroin, are primarily transported through existing border checkpoints via cars and trucks. Those checkpoints will be there whether the wall gets built or not.

Mexican drug cartels “transport the bulk of their drugs over the Southwest border through ports of entry (POEs) using passenger vehicles or tractor trailers,” the DEA writes in its 2015 National Drug Threat Assessment. “The drugs are typically secreted in hidden compartments when transported in passenger vehicles or comingled with legitimate goods when transported in tractor trailers.”

President Trump's interview with the AP's Julie Pace on April 21, was full of falsehoods and misleading statements. Here's a look at five of his worst whoppers. (Jenny Starrs/The Washington Post)

It doesn’t matter how high the wall is, in other words, because those ports of entry are the doors that the drugs already come through.

The image of drug smugglers who run drugs across remote stretches of the Southwest border is largely a fiction, according to drug policy experts.

“Smuggling drugs in cars is far easier than carrying them on the backs of people through a really harsh desert terrain,” said Vanda Felbab-Brown, a senior fellow at the Brookings Institution. “The higher the fence will be, the more will go through ports of entry.”

The Trump administration knows this. At an April hearing, Department of Homeland Security Secretary John F. Kelly acknowledged that illegal drugs “mostly come through the ports of entry.” At a separate hearing in February, the director of a customs border task force told lawmakers that “the Southwest land border POEs are the major points of entry for illegal drugs, where smugglers use a wide variety of tactics and techniques for concealing drugs.”

Smart border drug policy would mean improving those ports of entry instead of building a wall, says Adam Isaacson, senior associate for defense oversight at the Washington Office on Latin America: “If you really want to go after [the drug] problem, go after the ports of entry.”

Isaacson says the nation’s ports of entry need about $5 billion in improvements, including more personnel and better technology for screening people and vehicles. But he hasn’t seen that funding in the president’s spending proposals.

Beyond the ports of entry issue, the shortcomings of a wall as an anti-drug strategy are legion. “There are simply too many alternative ways of moving drugs into the country to justify a wall,” said Sanho Tree, director of the Drug Policy Project at the Institute for Policy Studies.

Drug tunnels. Catapults. Drones. Narco-subs that can carry several tons of product per trip.

“Contrast that with a human carrying 30 pounds on their backs trekking across the desert,” Tree said. “It makes little sense for traffickers to use the latter method.” He added, “The drug war has created the ultimate X Prize for criminals to find ever more ingenious ways to penetrate our borders.”

Beyond that, the economics of getting high suggest that in the unlikely scenario that the wall prevents significant quantities of heroin coming in, the result could be more overdose deaths.

Heroin cut with the powerful opiate fentanyl is a major driver of overdose deaths, Tree says. That fentanyl primarily enters the United States from China via the U.S. Postal Service.

“If the wall creates a heroin shortage, traffickers will adulterate remaining supplies of heroin with even more fentanyl,” Tree says. “This will cause overdose fatalities to skyrocket.”

Finally, many of the drugs killing people in the United States originate right here. Prescription painkillers are one of the chief drivers of the opioid overdose epidemic today, accounting for roughly as many deaths as heroin.

Those drugs aren’t being smuggled across the border, but are rather diverted from legitimate uses to the black market right here in the United States. A wall wouldn’t change that.

As ever, Trump remains undeterred by expert assessments and is committed to build the wall even if American taxpayers have to foot the bill. “The wall will stop the drugs,” he told the Associated Press last week.