It is lost to history whether Hadrian, Qin Shi Huang or Nikita Khrushchev ever uttered, “I will build a wall.”

But build they did, and what happened? The history of walls — to keep people out or in — is also the history of people managing to get around, over and under them. Some come tumbling down.

The classic example is the Great Wall of China. Imposing and remarkably durable, yes, yet it didn’t block various nomadic tribes from the north. History is full of examples of engineering thwarted by goal-oriented rank amateurs. But Donald Trump has promised to build a wall on the United States-Mexican border that he says will be big, beautiful, tall and strong, and he says Mexico will pay for it.