F our millennia ago, a Sumerian king, his frontier beset by nomadic tribes fleeing prolonged drought in their own lands, ordered the construction of the world’s first border wall: a 177km-long boundary laid in stone between the Tigris and Euphrates rivers. Since humanity’s earliest city-states and kingdoms arose in ancient Mesopotamia, walls, ditches and fences have defended territory, marked the edges of empires and projected political power across the void. But the world’s first border wall failed. It now lies buried beneath Iraq’s desert sands. Rome’s legions abandoned Hadrian’s Wall long ago, and the iron curtain’s razor-wire fences fell with the eastern bloc’s collapse in the late 1980s. The Ozymandian ruins of many such walls litter our ancient and modern landscapes, because for as long as humanity has built hard borders, people have inevitably found ways to cross, topple or simply bypass them.…