President Donald Trump on Thursday warned that he will impose tariffs on Mexico or close the US-Mexico border “if the drugs don’t stop, or largely stop.”

The threat is supposed to push Mexico into action in the international war on drugs, but more than anything, it shows Trump doesn’t understand why Mexico hasn’t stopped drugs from flowing into the US. It’s not Mexico’s apathy; it’s that the war on drugs, which Mexico is very much invested in, isn’t working.

Mexico has been trying very hard for years to stop the flow of drugs into America. The US has dedicated hundreds of millions of dollars and both military and law enforcement resources to Mexico for the cause, while Mexico has built up its own police and military to fight drug cartels and other trafficking organizations. Mexico has sacrificed a lot here; according to the Congressional Research Service, there have been 150,000 intentional homicides linked to the drug war in Mexico since the government there escalated its war on drugs in 2006.

But this clearly isn’t working. Today, drugs continue to flow freely into America — with illicit fentanyl, a synthetic opioid, and heroin linked to tens of thousands of drug overdose deaths in the US each year, as the nation deals with an opioid epidemic.

Trump’s comments put the failure on Mexico, suggesting that the tens of thousands of lives and hundreds of millions of dollars it’s spent in this effort aren’t enough. The reality is that a lot of the blame — if not most of the blame — falls on the US and a failure of American drug policy.

Why the war on drugs isn’t stopping drugs from Mexico

The reason drugs are coming to the US is not because Mexico just one day randomly decided to send them here. It’s because there’s a huge demand for the drugs in the US. That demand creates a major drug market for criminal organizations, which drug cartels readily tap into. In other words, it’s American demand that’s primarily driving the flow of drugs.

More supply can lead to more drug use, since easier access to drugs lets more people try illicit substances and get addicted to them. But there are a few reasons why the drug war, which is largely focused on cracking down on supply, hasn’t worked.

First is a concept known as the “profit paradox.” One of the primary goals of the drug war is to make drugs more expensive by limiting their supply; the idea is that a drug habit is much more difficult to sustain if drugs are more expensive.

But this also makes drugs immensely profitable. According to Jon Caulkins, a drug policy expert at Carnegie Mellon University, the war on drugs increases the price of drugs from production to sale in the US by as much as 10 times. This creates a paradox in which drugs are less affordable and accessible, but the drugs that are sold are enormously profitable for criminal organizations, despite all the trouble of smuggling the substances through Latin America and into the US. So drug cartels keep shipping drugs to the US, despite the risks of military and law enforcement action.

There’s also the “hydra effect”: When one source of drugs shuts down, another takes its place. The name comes from the mythological hydra, a beast that, in some versions of the story, grew another head when its previous one was cut off.

Since drugs are so lucrative, drug producers and traffickers don’t just cease to exist when governments detain or kill them. The business is so profitable that someone will always be there, willing to replace defunct organizations or leaders. So when Mexican and US officials captured El Chapo, someone simply replaced him as head of the Sinaloa cartel. And even if his powerful Sinaloa cartel collapsed, another drug trafficking organization would just take its place. There’s simply too much demand for illicit drugs for a criminal organization to not take advantage of it.

The effect is similar to the “balloon effect,” when cracking down on the drug trade in one area simply moves it to another area — sort of like pushing down on a balloon can simply move the air to other parts of the balloon. This effect has been documented all around the world, including Colombia, Mexico, Venezuela, Ecuador, and West Africa.

These concepts help explain why more aggressive anti-drug efforts haven’t worked. A 2014 review of the research by Peter Reuter at the University of Maryland and Harold Pollack at the University of Chicago found that while simply prohibiting drugs to some extent does raise their prices, there’s no good evidence that tougher punishments or harsher supply elimination efforts do a better job of driving down access to drugs and substance misuse than less aggressive efforts. So escalating the drug war — what Trump is calling for here — just can’t do much to stop the flow of drugs.

The real answer is more addiction treatment

These concepts are one reason border security experts say Trump’s wall won’t stop the flow of drugs into the US — on top of the fact that most drugs are shipped through legal ports of entry, not the illegal border crossings that a wall would attempt to prevent.

These failures are also a big reason drug policy experts point to addiction treatment as the main solution to the opioid epidemic. The thinking: If the US can’t stop the supply, it can at least reduce the demand by getting fewer people to use drugs.

In other countries, this has worked. When France opened up access to buprenorphine, a highly successful opioid addiction treatment medication, overdose deaths declined by 79 percent over the next five years as the number of people in treatment went up, a study in The American Journal on Addictions found.

But despite experts’ calls, Trump has not even proposed the tens of billions of dollars to treatment that experts argue is necessary to combat the opioid epidemic. Meanwhile, the US continues pouring tens of billions into its supply-focused drug war.

And now Trump is upset that it’s not working.