They say that strong walls make for good neighbours. But in polite society, making the case for border walls is to invite scorn. It puts you in bad company. Donald Trump wanted a big, beautiful wall along the border with Mexico. The communists divided Berlin with one – they started building it exactly 60 years ago today – and from Jerusalem to Belfast they are seen as symbols of division.

They certainly leave their mark. This week, we saw pictures of newly-discovered parts of Hadrian’s wall, which split Roman Britain from Caledonia – Scotland – as decisively as anything you’d see in Game of Thrones.

But the truth is that walls are back, if they ever even went away. In Europe, it’s happening from the Aegean to the Baltics. The Greeks are spending millions on a 25-mile-long 16-foot-high steel fence, complete with cameras, radar, drones and mini-airships. They’re also deploying long-range acoustic weapons, so-called sound canons, which deter migrants with noise.

Lithuania is so desperate to stop migrants coming over the Belorussian border that it’s run out of barbed wire for its new 400-mile-long border fence. It’s had to borrow some from neighbouring states.

And then there’s Turkey, not strictly European, but building walls with gusto nonetheless. Who are they trying to keep out? Well, here’s the real shocker. Even before the Taliban started going through rural Afghanistan like a dose of salts, it was Afghans who a lot of these new border walls were being built to keep out.

Shia Hazaras from Afghanistan reportedly make up about half of the migrants in Eastern Greece’s refugee camps. I was there on the Greek border in 2015 when hundreds of thousands of migrants heeded Angela Merkel’s invitation to come to Germany for a new life. It was obvious to me then that, although the mainstream media narrative was that the asylum seekers were predominantly Syrian, a very great many were not.

Once the dust died down this became clear. Two thirds of the 2015 migrants who made it to Sweden, for instance, were Hazaras from Afghanistan. It was Merkel who made that happen. And she won’t do it again. Once bitten, twice shy. The German Chancellor now says her borders won’t be thrown open for a new wave of migrants, this time Afghan, not Syrian.

Merkel thinks a negotiated peace deal between the Afghan government and the Taliban might stop an exodus of refugees. She’s already behind the curve. In neighbouring Austria, home to a large Afghan immigrant population, the Army are being sent to the Eastern border.

When Victor Orban did that in 2015, and started building a huge new border fence, he was vilified. I was at a border crossing, broadcasting live between Hungary and Serbia when the migrants tried to force their way in, hurling rocks at riot police. For me it produced an Emmy award but also, as the months passed, quite a lot of introspection.

I can’t speak for others, but looking back, I don’t think I fully grasped what was happening, or if I did, I lacked the courage to tell the story faithfully. But that was then, this is now. Here’s the essential arithmetic. There are at least 100,000 Afghans in Britain, and that diaspora will act as a magnet to migrants. In Afghanistan, where the median age is 18, there are now millions of young people who’ve had their horizons expanded by 20 years of relative peace, who are – quite rightly – considering their future.

They’ve been let down by Joe Biden, whose decision to abandon them to the Taliban is an epic mis-judgement that, had Trump had done it, you wouldn’t have heard the last of. In time, we may come to see Biden’s decision as even more short-sighted than Merkel’s six years ago. 

It may not be too late. Britain is sending an emergency force to Afghanistan. But they won’t stay. And even if UK forces did, as I believe they should, we are probably too few to turn the tide. So get ready for another migrant exodus. And prepare yourself for the sight of more border walls in Europe where they can be built, and many more boats on the waters of the English Channel, where they can’t.

Colin Brazier is a GB News journalist and host of Andrew Neil at 8pm Mon-Fri. GB News is on Freeview/YouView 236, Sky 515, Virgin Media 626 and Freesat 216. Also online and via mobile app.