“Build that wall,” they chanted at the Donald Trump campaign rallies last year. And build that wall he will. On his own terms, as the first step in honouring a solemn commitment made to the American people, Mr Trump is delivering. The initial executive order issued, he will then move on to the vast logistical challenge of building a physical obstacle around 2,000 miles long on the Mexico border.

Or not, as he has also sometimes indicated that it need not literally be a wall. Still, much of it will be a solid, high, forbidding barrier plonked right on the border with America’s increasingly brow-beaten southern neighbour. Mr Trump will also have to secure federal finding form the US Congress for the wall – and senators and congress members know there are better ways of spending about $20bn. Mr Trump will also have to reassure his supporters in Congress and in the country at large that he will indeed make the Mexicans pay for the wall, if only indirectly through a variety of fiscal measures – tariffs included.

So the political and financial path to building the wall is clear enough, and it carries with it obvious formidable practical challenges. It could be done, if enough federal money and effort was thrown at it: after all, America put a man on the moon and developed the world’s largest economy. None of that, though, makes this anything other than a reckless, crazy and, in the end, useless monument to a sort of moment of populist madness in America’s political history – whether it is visible from space or not. Trump’s wall will for decades stand as a reminder of what went badly wrong with American democracy in the early decades of the 21st century. That will, though, be about the limits of its usefulness.

For the Mexican wall to be truly effective, it will require the same kind of infrastructure and manpower that has characterised similar exercises in the past and present. The Berlin Wall, for example, had an elaborate network of watchtowers and was permanently patrolled. The wall Israel is building – illegally – around its claimed territory is also supported by the presence of the Israeli defence force. The DMZ between the two Koreas is also bristling with troops and armaments. All would be dwarfed, though, by the Trump wall, and the reality is that no American administration, not even one as bull-headed as Mr Trump’s, would commit to the ruinous expense of operating permanent impenetrable military operation across its length.

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It would, indeed, be cheaper to pay any Mexican would-be migrants not to cross the border than to divert a substantial proportion of America’s military to this absurdist exercise. Just because the people voted for it does not make it a good idea.

As a wedge of concrete stretching unsupervised across the continent, the Trump wall would be comparatively easy to tunnel under, get across or simply go around by the eastern or western seaboards, a sort of comical version of the Maginot Line the German army found so irrelevant in 1940.

As we in Europe witnessed these past few years with the wave of desperate refugees traversing the Mediterranean, taking their lives in their hands, there is no end to imagine the courage and despair that will drive hitherto sane humans to put their lives and those of their families at risk if they are fearful enough. Mexican and other migrants will find easier ways to get into the United States, if needs be on a passenger jet: forged documents, bogus stories about families, collusion by corrupt officials on either side of the border. Mr Trump’s wall, in other words, is unlikely to prevent illegal migration, even if manages to reduce it temporarily. The flow will redirect to the line of least resistance, as it always does.

Past presidents have other physical reminders of their achievements and the nation's gratitude to them: space centres, peace centres, airports, dams, schools and hospitals. The greatest have grand memorials in the national mall. Mr Trump, aside from his garish hotels, will have the world’s longest and most useless strip of concrete, scarring the landscape and disrupting nature itself.