Even by his own vain, erratic, rambling, bombastic standards, the latest Trump tweets are astonishing. I wouldn’t wish to misquote the president of the United States, so here they both are, in all their succinct majesty:

“On June 10th, the United States will impose a 5 per cent Tariff on all goods coming into our Country from Mexico, until such time as illegal migrants coming through Mexico, and into our Country, STOP. The Tariff will gradually increase until the Illegal Immigration problem is remedied..”

“…at which time the Tariffs will be removed. Details from the White House to follow.”

So that’s how America governs itself these days. And I thought Britain was a mess.

It’s difficult to know where to start with this. Why not with how idiotic, illogical, counter-productive, arrogant, ignorant and economically illiterate it all is? Why impoverish Mexico when all it will mean is more economic migrants trying to get into America? What then, Mr President, will you do?

Ah yes, we know the answer, because “the details” are that the tariffs will keep going up in 5 per cent steps until they hit 25 per cent. Then, when Mexico’s agricultural and industrial exports to its largest market have been decimated, and only then, will Trump stop increasing the punishment meted out to Mexico. At which point America’s exports to Mexico will also have been destroyed – because Mexicans can’t afford them and because of retaliatory tariffs – and everyone will be the poorer. A lesson in the benefits of free trade and the evil of poeticism right there, and one that will be blithely ignored by Trump.

The truth is that the immigrants are not coming into the US because the Mexicans aren’t bothering to patrol their own borders. To go back to an old Trumpism: “When Mexico sends its people, they’re not sending their best,” he said then. “They’re bringing drugs. They’re bringing crime. They’re rapists. And some, I assume, are good people.”

First, Mexico doesn’t “send” anyone. The migrants are, predominantly, not coming from Mexico at all, but from the nations in crisis to her south, most obviously from Venezuela.

If the president wants to break Mexico’s economy in the same away that Chavez and Maduro have wrecked Venezuela, then he should not be surprised if his tariff war on Mexico only ends up with Mexicans joining the Venezuelans and others trying to smuggle themselves into the US.

The underlying fallacy in all of Trump’s thinking is that controlling migration is all about physical borders, walls and draconian punishments. They help, of course, but the reality is that he could implement a shoot-on-sight policy across the border and even that would not stop desperate human beings from fleeing to the safety and opportunity they see in America.

It has always been thus. Maybe Trump should ask himself why his grandfather and his wife and millions of others over the centres came to America. Was it because of lax border rules or because they wanted to make a better life? How so many entered illegally? How many made arduous and painful journeys across continents to reach freedom, just as now? The Vietnamese boat people? The refugees from Cuba?

The most effective way to push migration from the south to the US onto a downward trajectory is to even up the living standards either side of the Rio Grande. That is what free trade and free movements of investment – globalisation – does. It’s what the Nafta tried to do, before Trump scrapped it.

Making Mexicans poorer does not make America secure and great again. When will the madness end?