Ironically Constantius Chlorus was destined to be the father of another British-based usurper. Constantius had been one of the Tetrarchy, a college of four emperors who ruled jointly - a system optimistically designed to stabilise the imperial succession that was never likely to survive the abdication of its founder, Diocletian. On Constantius’s death the British army elevated his son at Eburacum. After a complicated series of political, dynastic and military conflicts, the son had become master of the west, though he still faced his rival in the east. But, of course, as a winner he was no longer regarded as a usurper.

Few of Thalius’s friends had studied history as he had, and few knew that poor, charismatic, doomed Carausias had been only the latest of a series of usurpers across the empire. The first British-based usurper had been a governor, African-born Clodius Albinus, who, seventy years after Hadrian, took the British garrison to the continent, only to be destroyed by the emperor Septimius Severus. Severus had split the province in two, so that no governor could ever be so powerful again. A century later Constantius Chlorus had split the provinces again; there were now no less than four Britains. But the years between Severus and Constantius had seen little peace.

Thalius had concluded you could trace all this instability at the top of the empire back to Hadrian and his Wall, completed nearly two centuries in the past.

The Wall itself had proved a durable limit to Rome’s ambitions. Though Severus, conqueror of Clodius Albinus, had ventured beyond the Wall, reaching the furthest point of the highlands, his campaign dissipated on the bleak high ground - just as had Agricola’s before him. The Romans had never again attempted to conquer the far north.

But there were consequences of the end of expansion. With no new provinces to plunder, the empire’s only income was the taxes and levies it raised on its peoples. Meanwhile beyond the static frontiers, even in Caledonia, the barbarians had the chance to form new and more coherent federations, and were becoming an increasing threat.

So while the empire’s acquisition of wealth had declined, the cost of defending it was rising - and taxes inevitably rose, generation on generation. The gentry fled their expensive responsibilities in the towns for grand estates in the country, while the poor were driven into evasion, criminality and destitution.