Rory Stewart, wanderer, writer, once a soldier, briefly deputy governor of an Iraqi province, now a Member of Parliament and a junior minister, has a roving, enquiring mind, which makes him on the page (the only place I know him) most agreeable company. Ostensibly this book is an account of a walk along Hadrian’s Wall (a mere doddle compared to Stewart’s earlier walk across Afghanistan), but he ranges far beyond this. He is (sort of) accompanied by his remarkable nonagenarian father, Brian, who travels by car and meets him at stopping places along the way. Both have the ‘satiable curiosity’ that Kipling ascribed to the elephant’s child. Brian, a proud Scot clad in tartan trews (though he sometimes claims to be Irish), questions Stewart about his findings, occasionally playing deaf when he doesn’t like the answer. Earlier in his life, Brian served as a colonial officer and in the intelligence service; at one point he commanded a battalion of the Black Watch (in an oddity relevant to Stewart’s speculations about the nature of Britain and Britishness, this particular battalion of the famous Scottish regiment was actually raised on Tyneside and composed of Geordies).

The British Empire is long gone, ‘one with Nineveh and Tyre’ (Kipling is unavoidable in the context). Nowadays children learn more about its vices than its virtues. So Hadrian’s Wall is a good place to think about empires and their significance. Why was the wall built, and why