In his new book, James Crawford sets out to challenge the glib assumptions behind phrases like ‘good fences make good neighbours’ and geopolitical nostrums like ‘if states had “natural boundaries” along rivers or mountain ranges, wars would be far less common’. From his yomp across glaciers searching for the watershed in the Dolomites that separates Italy from Austria to his forays into the Middle East and travels along the Mexico–USA frontier, Crawford finds borders more difficult to define and less conducive to what their architects intended than we might think.

But he also finds charm in delineation. In his native Scotland, he follows the route of the Antonine Wall, which wasn’t made of stone like its southern counterpart built by Hadrian. What the turf wall built by Hadrian’s successor lacked in durability, it made up for in carved stone markers unprecedented along the Roman Empire’s borders. Did the Romans finally recognise that their empire had reached its limits and want them commemorated forever – or at least to be found in situ by archaeologists in a barely conceivable remote future?

Vanished boundaries exert a fascination. Crawford notes the fad for collecting graffitied bits of the Berlin Wall after its collapse. The eastern side had been too dangerous to graffiti before 9 November 1989, but after that day the Ossies, newly filled with free-market zeal, spray-painted their dull grey