Meanwhile after Constantine’s death fratricidal war between his three competing sons brought renewed turmoil at the heart of the empire. Britain threw up usurpers, such as one Magnentius, who had killed one of the sons, only to be defeated by another, at Mursa in Pannonia. The empire once more became an arena of conflict between strong men and their armies. Then, when Constantine’s treaties were repudiated by one barbarian group, the Visigoths, decades of simmering conflict finally led to battle at Adrianopolis - a battle the Romans lost, at a terrible cost.

Times had changed, and a weakened empire could no longer afford such bloodletting, and after Mursa and Adrianopolis the Roman army had never been able to recover its strength. The emperors employed barbarian federates in their armies, and still more alien peoples settled within the borders - following Constantine’s precedent, but now the process was all but uncontrolled. Thus the army was barbarised, and the empire was hollowed out by foreign polities.

The British felt increasingly exposed, and its elite and officer corps threw up one usurper after another - roughly one a generation, as young rebels dreamed they could do better than their fathers. And as soon as he took power each of them headed overseas, taking yet more troops with him from the British garrison. One, grandly named Magnus Maximus - ‘the Great, the Greatest’ - killed another emperor, and had the dubious honour of being the first western ruler to order the execution of a Christian heretic.

But the troops stripped from the British garrison by each failed rebellion, and indeed by each officially ordered transfer to reinforce the centre, were never returned. Without the army to enforce collection the taxation system began to break down. People hoarded their coins in the hope of better times, and a barter economy sprang up. But as demand dwindled there was much destitution, and Ambrosius, by now a young priest, found himself ministering to the starving poor.

Then, one New Year’s Eve, a vast horde of Alans, Suebi and Vandals crossed the frozen Rhine, and pushed their way into Gaul and Iberia.

Britain was cut off. Now no money was coming from the central treasuries to pay the remaining British troops. The imperial standard still flew over the forts and towns, but there was little left of the Roman army beneath it. After a succession of bloody, panicky coups a low-born general who styled himself ‘Constantine III’ put together yet another ragged army, and just like his predecessors immediately crossed to the continent. Constantine was the last throw of the dice, the last British attempt to save Britannia.