Donald Trump’s proposed Wall of Mexico. Theresa May’s Wall of Calais. Benjamin Netanyahu’s plans to extend the wall between Israel and the West Bank by building a barrier round the entire country.

Say what you like about stagnation in the global economy, wall builders seem to be in for a boom. The only question is whether the barriers will do more harm than good to those protected by them.

History is littered with examples of walls built with a defensive, embattled mindset that failed in their primary purpose. The Great Wall of China, built to keep out raiders from the north, was not enough to stop the Qing armies breaching the barrier and deposing the Ming dynasty as rulers of China. The Maginot Line, constructed by the French after the first world war to keep Germany at bay, did not prevent Hitler invading France in 1940 through the novel tactic of going round the end. The Berlin Wall, put up to isolate the virus of western capitalism from the Soviet bloc that surrounded it, became the most well-known metaphor for the collapse of communism.