Mercia had been the dominant power in what is now midland England since the seventh century, and Welsh kingdoms had paid tribute to it for two hundred years. In 853 the West Saxons of Wessex had even sent help to enforce that arrangement. Then the Vikings arrived. The Great Heathen Army landed in 865, ravaged Northumbria and East Anglia, and in 874 drove King Burgred of Mercia into exile. By 877 the Vikings had partitioned Mercia, ruling the eastern half themselves and installing the puppet king Ceolwulf II in the west. Gwynedd, meanwhile, was also under Viking attack: in 877 Rhodri Mawr was driven out of his own kingdom, returning the following year only to be killed by the Mercians, who were still trying to maintain their old hegemony over Wales. King Alfred's victory at the Battle of Edington in May 878 relieved Mercia's Viking problem. Ceolwulf died or was deposed in 879, and Æthelred took over as Lord of the Mercians.

Two years into Æthelred's rule, the Mercians invaded Gwynedd again. The sons of Rhodri Mawr - Anarawd, Cadell, and Merfyn - met them by the River Conwy. The exact location is unknown. A late source, the seventeenth-century History of Wales, says the Welsh army camped near the town of Conwy at a place called Cymryt - perhaps the modern Cymryd - and that Anarawd 'made gallant resistance against the pressing efforts of the Saxons' there. The early Welsh annals are more compressed: the Welsh won, and called the battle Gwaeth Cymryt Conwey, or Dial Rodri - 'Vengeance for Rhodri.' According to a thirteenth-century Welsh genealogy the Mercian leader was 'Edryd Long-Hair,' almost certainly Æthelred himself. What happened in the field - cavalry charges, shield walls, river crossings - is lost. What remains is the result and the name.

Mercia abandoned its claim to lordship over north Wales after 881. Æthelred turned his attention south and east, toward the Welsh kingdoms of Glywysing and Gwent, but those kingdoms - according to Alfred's biographer Asser - then appealed to Alfred for protection 'driven by the military power and tyranny of Ealdorman Æthelred and the Mercians.' By 883 Æthelred himself accepted Alfred's lordship, marking the beginning of the political integration that would, generations later, become the Kingdom of England. The historian Thomas Charles-Edwards has argued that the Mercian submission to Alfred was not just the consequence of Alfred's Viking victory at Edington but also of a second, more distant defeat by the Welsh on the Conwy three years later. Both battles, in different ways, made an English kingdom possible.

Anarawd ap Rhodri - now ruling in his father's place - briefly allied Gwynedd with the Vikings of Northumbria after the battle. Asser, writing for Alfred, attributes the southwestern Welsh kingdoms of Dyfed and Brycheiniog seeking Alfred's protection to 'the tyranny of Gwynedd' under Anarawd. But the Viking alliance brought him no benefit, and Welsh annals record Viking raids on Gwynedd continuing into 892. Anarawd eventually broke the alliance and, like Æthelred, submitted to Alfred. The same blood feud that had brought a Welsh army to the Conwy in 881 ended with Welsh and Mercian alike orbiting around Wessex by the close of the century.

Walk along the Conwy estuary today and there is nothing to mark the battle. No memorial, no information board, no certainty of where it happened. The site survived the eleventh-century Norman incursions, four centuries of medieval lordship, the Edwardian conquest that built the great castle at Conwy four hundred years later, and a thousand years of subsequent farming. The battle's victors are commemorated in the names of villages and rivers across north Wales; the battle itself is a footnote in the history of how a Welsh blood feud helped tip the balance toward an English unification that would, three and a half centuries later, conquer the Welsh kingdoms entirely. History rarely runs in straight lines.

The battle is traditionally located near Conwy at a place called Cymryt, possibly at modern Cymryd on the west bank of the river. Coordinates 53.28 north, 3.83 west bracket the likely area. The Conwy estuary is a wide tidal river running south to north between the Carneddau range and the Creuddyn peninsula. Best viewed at 2,000 to 3,000 ft AGL flying along the river. Nearest airports: Caernarfon (EGCK) twelve miles west, RAF Valley (EGOV) on Anglesey, Hawarden (EGNR) east toward Chester.