Tear down this wall” demanded Ronald Reagan in Berlin in 1987. “Build the wall” demands Donald Trump, the man poised to take over Reagan’s party by winning the Republican nomination for the US presidency in 2016.

While America is still debating Mr Trump’s demand for a “great, great wall” along its border with Mexico, Europe has already entered the wall construction business. The EU’s panic over the “migrant crisis” is leading to a multiplication of new physical barriers and checks in Europe, to block the passage of would-be refugees.

Once again, there are some painful historical ironies. The first breaches in the Iron Curtain in the summer of 1989 came when the Hungarian government removed the electric fencing that separated its country from Austria — a decision that set off a train of events that culminated in the fall of the Berlin Wall a few months later. A quarter of a century on, Hungary has once again been a trailblazer, but this time in the opposite direction. When Viktor Orban, Hungary’s prime minister, built a razor-wire fence along his country’s frontier last summer, to deter would-be refugees, he was roundly denounced. A few months later, an Orban-style fence has just been built along the Greek-Macedonian border, and frontier controls are being tightened across Europe.