The second of May marks the first anniversary of the death of Osama bin Laden. I've been away for five months, writing a book about Roman Britain, and, while the orchestrator of 9/11 hasn't exactly been at the front of my thoughts, he did come to mind because of something that Mary Beard said in a book review in the Sunday Times the other week. The book in question was Sam Moorhouse and David Studdard's excellent The Romans Who Shaped Britain, and Beard's memorable aperçu was: "Britain was Rome's Afghanistan".

Like any such neat phrase, of course it's too neat. And yet, as soon as I read it, I could see what she meant: Britain was a thorn in the side for Rome, requiring a disproportionate number of troops and proving a huge struggle to properly subdue. It wasn't fully conquered until nearly 40 years after the initial invasion, when Agricola won the Battle of Mons Graupius in northern Scotland; and even then the Highlands were let go almost at once. But I couldn't help too being reminded of Caractacus, the Iron Age British leader who fought against the Romans in AD 43 and, despite being assiduously pursued by the Roman war machine, managed to slip away from their grasp, head west, and hold out for seven years in his lair in the Welsh mountains, orchestrating resistance. When finally the Romans caught up with him – defeating him in battle at a north-Wales hillfort – he managed to slip away again, and sought refuge with the northern English Brigantes tribe. Which was a bad idea: Queen Cartimandua, a Roman ally, handed him over to the Romans.

Claudius, with Caractacus taken alive, was able to do what the US couldn't with Bin Laden: lay on a show – carefully stage-managed to make the capture reflect as gloriously as possible on himself. As the historian Tacitus has it in his Annals, Caractacus had become a famous name in Italy – and when the Romans heard of his capture, there was a belief, entirely false of course, that Roman troubles in Britain had ended. At any rate, "There was huge curiosity to see the man who for so many years had spurned our power," as Tacitus put it. So a parade was organised, with Caractacus and his wife and children forced to process through the city. Caractacus dared address the emperor and, according again to Tacitus, made an extraordinary speech, pointing out that under different circumstances he might have been welcomed to Rome as a friend, rather than dragged there as a captive. He then pointed out to Claudius that no one would have thought his capture a great achievement if he had surrendered immediately. And he added (I paraphrase, but only slightly): "Everyone will forget this, now, if you kill me. What would be much more memorable would be if you spared me. Then I will be an everlasting memorial to your clemency." It was a startlingly modern-sounding appeal to PR, and it worked: Claudius pardoned the Briton and his family – who lived out (we have no reason not to believe) the rest of his life in Rome.

Tacitus records his encounter with Claudius not, in fact, to demonstrate his humanity, but to show how a barbarian from the furthest reaches of the known world could get his own way, using that most Roman of skills, rhetoric. The episode lives on as not an everlasting memorial to Claudius's clemency, but to Caractacus's cunning. Which goes to show that sometimes it is shrewder for great empires to take their worst enemies dead, rather than alive.