In 1961, Soviet Premier Nikita Khrushchev gave the East German government permission to build a wall to seal the border between East and West Berlin in an attempt to stop the overwhelming flood of refugees fleeing the repression and economic hardship of the communist East for the freedom and prosperity of the capitalist West.

For the next 30 years, the Berlin Wall stood as a stark symbol of division and repression. When it finally came down in 1989, images of gleeful Berliners hacking at the steel-and-concrete behemoth with hammers became a powerful symbol of unity and the triumph of freedom.

On Wednesday, President Donald Trump signed an order directing the Department of Homeland Security to begin construction on a giant new wall between the US and Mexico — not to keep people in, but to keep people out.

And on Friday, International Holocaust Remembrance Day, Berlin Mayor Michael Müller issued a powerful message warning Trump “not to go down that that road to isolation and ostracism”:

Berlin's mayor just issued this message for Donald Trump. It's quite something. https://t.co/gIIOei6ttT pic.twitter.com/B8SgYZhhJ0 — Jeremy Cliffe (@JeremyCliffe) January 27, 2017

Trump seems likely to push ahead with his plans, even though many Republicans on Capitol Hill question whether building such an enormous and costly wall is feasible (and where to find the money).

That means Müller almost certainly won’t be able to stop the wall from going up. But his message at least offers a reminder that the costs won’t simply be financial.