Trump displayed his usual look, with his red tie tied tightly around his shirt collar. The occasion was his first visit to California, last March. The chosen location was the San Diego desert, at a short distance from the United States border with Mexico. The president was holding in his hands a poster which compared the porous Californian border of the Nineties with the current one, fenced off and under tight surveillance.

For Trump, San Diego is the proof that even the "worst" wall – his, he guarantees, will be of a different calibre and even more effective– can help to restore law and order along the border. It has to be said that San Diego is very different today to what it was like thirty years ago: at the time it was one of the most crowded entry points for illegal migrants, mainly Mexicans, while now it has become one of the hardest points to cross for anyone wishing to enter the United States. Last year in these parts the border police arrested 96% less people compared to the figures for 1986. This is partly due to gradual changes in migration patterns and economic and demographic transformations in Mexico, but there's no doubt that the tighter controls have played their part.

The impenetrable barrier Trump means to build along the southern border is not only designed to block the access of migrants. It's also supposed to stop drug trafficking. The president has often mentioned the wall as the solution to the opidemic, the opium epidemic that has ledto the death from drug overdose of 42,000 Americans in 2016 alone. But the example provided by San Diego, which Trump has pointed to as a model worth imitating, should convince him to revise his strategy: the San Diego fencing has undoubtedly discouraged illegal immigration, but it hasn't stopped the flow of narcotics. It would appear instead that most of the substances reaching the American territories come right through here.

In October 2017 Donald Trump declared that the opioid crisis had to be considered a national health emergency. And, if the figures are anything to go by, it is exactly that: between 2000 and 2015 over three hundred thousand American citizens have died of overdoses brought about by excessive intake of heroin or pain killers with a high opium content such as fentanyl. This averages out at around one hundred cases a day. According to the Centre for the prevention and control of illnesses, a United States health monitoring organism, between 2014 and 2015 alone the mortality rate linked to heroin increased by 20.6%, while that associated with synthetic opioids climbed as high as 72%. The problem is particularly serious in Ohio, New Hampshire and West Virginia, where the economic crisis and impoverishment have favoured the consumption and dependence on pain killers and ultimately heroin.

The White House's plan against opioids seems intent on dealing with the crisis more as a security problem than a health issue: so the focus is more on punitive measures rather than prevention and medical treatments. The president has for example floated the idea of introducing the death penalty for certain types of dealers and drug traffickers, and is insisting on the need to raise the wall in front of Mexico to keep the drugs away from the American territory.

it has to be said that most of the heroin circulating in the United States is of Mexican origin: approximately 93% according to the latest DEA estimates. The United States anti-drug agency also believes that even a large amount of the fentanyl – with the exception of the stuff channelled through the legal market – is smuggled over the southern border after being imported from Asia. Trump is right to accuse the Mexican criminals of "injecting drugs and death into the US", as he writes on Twitter, but he's wrong to believe that a wall will help to stop this traffic.

As it happens, heroin and cocaine, or drugs like fentanyl do not reach the United States across the remotest points of desert shared with Mexico. Instead, they arrive through the main ports of entry, the border passages between the two nations, mingled in with the movement of legal vehicles, goods and people: the goods are compressed in packets and hidden in secret compartments in cars and trucks, or in the clothes and on the bodies of cross-border workers.

That's what is apparently going on right now in San Diego. In spite of the four and a half metre high barrier, this is where the custom's police confiscated three quarters of the fentanyl spotted crossing the American border last year. This would seem to indicate that the wall has not discouraged smuggling, which instead is interwoven with the vehicles and the pedestrians that are a natural part of this vast metropolitan area that unites this city to its twin city Tijuana. Increasing customs checks would however increase waiting times and therefore transportation costs, thus damaging the very profitable bilateral trade between the United States and Mexico.

A clear-headed foreign policy would offer Trump the chance to assuage the opioid crisis, by at least confronting the aspects of the problem related to Mexican organised crime. Trying to push through the wall project, with all the rhetorical impetus it has been granted during the electoral campaign, is instead totally ineffective but also counterproductive from a diplomatic point of view: raising a physical barrier along the border – or saying he intends to – means also raising a barrier of hostility and diffidence between administrators in Washington and Mexico City. At a time when they could even work for the common good.

Just as Mexico helps to fuel the opioid epidemic in the United States, the demand for drugs in the United States has heightened the level of violence in Mexico. It’s a complex and bidirectional problem, that requires an approach based on mutual collaboration. The level of cooperation against drug trafficking between the two neighbours is in actual fact very high, but Trump's aggressive style risks slowing any further progress. Trump is unpopular with Mexican public opinion which also is very sceptical of any kind of partnership with Washington because it fears interference with its national sovereignty. For the Mexico City government it's therefore very hard to justify new military understandings with the United States. And the new president – the left wing nationalist Andrés Manuel Lòpez Obrador, who will be instated in December – could even decide to review existing cooperation mechanisms, which would make things even harder.

Since the launch of the Merida Initiative (a bilateral security agreement) in 2008, the United States have sent aid and equipment to Mexico worth approximately 2.9 billion dollars. During the tax year 2018 Congress has earmarked 145 million dollars to fight the production and distribution of drug traffic beyond the Rio Grande. The main goal of the Trump administration is to destroy the opium poppy fields in Guerrero, a poor, violent and mountainous state in South-Western Mexico where growing this plant is often the only source of income. To try and fight the illegal economy without bringing the farmers to their knees, LòpezObrador is thinking of legalising part of the opium production for pharmaceutical purposes. The White House has already let it be known it does not approve.

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