By the early seventh century, the island of Britain was being torn between two halves of itself. The Brythonic Christian kingdoms of the west, Welsh-speaking and rooted in the old Roman world, still held the spine of mountains running from Cornwall to Strathclyde. To the east, the pagan Anglo-Saxon kingdoms were expanding, their warbands pushing the Britons further into the hills. Chester sat on the fault line. Once the legionary fortress of Deva Victrix, it had become a frontier - close enough to the Welsh kingdoms of Powys and Gwynedd to matter to them, close enough to Anglo-Saxon Mercia and Northumbria to matter to them too. When Aethelfrith led his army south, he was not just hunting a Welsh force. He was severing the long thread of late Roman Christian Britain from its mountain heartland.

The monastery at Bangor-is-y-Coed, a few miles south on the River Dee, was one of the great Christian houses of Britain. Bede claimed it sheltered more than two thousand monks, divided into seven communities each numbering three hundred, all of whom lived by the labour of their own hands. Bede had no love for them. They had refused to accept the authority of Augustine of Canterbury and the Roman mission he led. They prayed in their own tradition, kept Easter on their own calendar, and ignored the new English church taking root in Kent. To Bede, writing a century later, what happened at Chester was a kind of grim divine justice - the Britons had refused to share their faith with the Saxons, and so the Saxons would meet them on the battlefield. Reading it now, the cruelty is harder to file away. Twelve hundred unarmed men, by Bede's count, were cut down because they believed their prayers might work. They had families and teachers and the small daily work of copying texts and tending gardens. Their lives ended in the mud because a king found their devotion threatening.

There is a strain of historical writing that tries to explain the massacre as tactics. Strike the praying men first, the argument runs, and the watching Welsh army will be thrown into spiritual disarray; their morale will break before steel touches steel. Maybe. Maybe Aethelfrith simply believed what he said he believed, that men praying against him were combatants. Either way, what followed was a hard fight. Welsh leaders Selyf Sarffgadau of Powys and Cadwal Crysban of Rhos died on the field. Gwion ap Cyndrwyn of Pengwern fell with them. Some sources suggest King Iago of Gwynedd may also have been killed. Their names survive in chronicles, scratched onto parchment by men who came after, but the rest of the dead - the warriors of the bodyguards, the levied farmers, the monks themselves - went into the earth nameless.

Historians once read the Battle of Chester as the moment Wales was cut off from the Brythonic kingdoms of the Old North - Rheged in modern Cumbria, Strathclyde further beyond. That view has softened; the sea was always the highway, and archaeology shows little Anglo-Saxon settlement in this corner of Cheshire during the pagan period. The land-bridge story may be more memory than fact. What did happen, certainly, is harder to map but no less real. Bangor-is-y-Coed never recovered. Its great monastery faded. The annals of Ulster remembered the slaughter, calling it strages, the slaughter, and the word echoed for centuries in Welsh memory as a particular kind of wound: the day a king killed men who had laid down their weapons before he arrived.

There is no definitive site marker, no preserved earthworks, no obvious place to stand. The battle may have been fought near Heronbridge, just south of modern Chester along the Dee, where archaeological digs have turned up skeletons of men from the period killed by weapon trauma. Or it may have been further south still, closer to Bangor-is-y-Coed itself, as the Welsh chronicle Brut y Brenhinedd suggests. From the air, the landscape gives nothing away. Pasture, hedgerow, the slow brown line of the Dee. Cattle. A few stone churches. The brutality of that morning has been absorbed into ordinary English countryside, and only the names in the chronicles still carry the heat of it.