"Tear down this wall," demanded President Ronald Reagan in Berlin in 1987. "Build the wall," demands Mr Donald Trump, the man poised to take over Mr Reagan's party by winning the Republican nomination for the United States' presidency later this year.
While the US 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 European Union's panic over the "migrant crisis" is leading to a multiplication of new physical barriers and checks to block the passage of would-be refugees.
Once again, there are some 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 is once again a trailblazer, but this time in the opposite direction. When Prime Minister Viktor Orban built a razor-wire fence along Hungary's frontier last year, 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.
The journey from Mr Reagan to Mr Trump - from tearing down walls to putting them up - says a lot about the West's journey from confidence to fear over the past 30 years. There are many reasons for this new demand for barriers between the West and the rest. The most obvious and direct cause is the fear of mass immigration from what used to be called the Third World. But, beyond that, there is a broader loss of faith in the West's ability to engage successfully with the outside world.
Even before the migrant crisis, anti-immigration parties were on the rise across Europe. They are almost certain to gain strength amid the present panic. Beyond the fears about mass migration, however, there is also a crumbling of some of the ideas that have underpinned Western engagement with the outside world since the end of the Cold War. The first principle is the promotion of a "globalised" economy through the removal of barriers to trade and investment. The second is a willingness to contemplate foreign military intervention in the world's trouble spots.
These two ideas - globalisation and liberal interventionism - were indirectly linked. The best solution to poverty and instability in the non-Western world was (and is) routinely said to be economic growth, through increased trade and investment. But, after the fall of the Berlin Wall, Western powers also became more willing to contemplate military intervention to "stabilise" failed states and troubled regions that had proved impervious to the magic of globalisation.
Yet, after 25 years of governments running these policy experiments, Western voters seem increasingly sceptical about both globalisation and liberal interventionism. In the aftermath of the Iraq and Afghan wars, there is very little appetite for further large-scale Western military intervention in the Middle East. All the Republican candidates in the US presidential race are prepared to pile into President Barack Obama for "weakness" in Syria, but none wants deployment of ground troops. Similarly, while there is anguish inside the European Union about the influx of refugees from Syria, there is no discussion of sending troops to end the conflict that is driving the refugee flows.
New trade agreements are also going out of fashion. Mr Trump is proposing not just to build a wall along the Mexican frontier, but also to impose swingeing new tariffs on US manufacturers based in Mexico. Even Mrs Hillary Clinton, the Democratic front runner, is making protectionist noises on the campaign trail.
Mainstream politicians will continue to argue that building barriers is not the solution to the problems of the world or the West. But they are in danger of finding that their voters have stopped listening.
THE FINANCIAL TIMES