The digging of Offa's dyke in the 8th century, as the effective border between Anglo-Saxon England and Celtic Wales, formalizes a situation which has existed for a century and a half. Victories near Bath (in 577) and near Chester (in 613) have brought the Anglo-saxons to the Bristol Channel and the Irish Sea, restricting the Celtic tribes to the great western peninsula protected by the Welsh mountains.

In this enforced seclusion lies the beginning of the Welsh identity. The region is called Wales from an Anglo-Saxon word wealas, meaning 'foreigners'. Similarly the beleaguered Celts begin to call themselves cymry ('fellow-countrymen'), naming their shared territory Cymru.

Like their Celtic neighbours over the water in Ireland, the Welsh have a strong early tradition of Christianity. But St David, the patron saint of Wales, is a more shadowy figure than Ireland's St patrick. Little is known of him except that he founds several monasteries in the late 6th century and makes his own ecclesiastical base at Mynyw, now known after him as St David's.

The Welsh retain their Celtic version of Christianity much longer than the English. The Roman calculation of Easter is not accepted in Wales until 768, more than a century after the Synod of whitby.

By the middle of the 9th century the Welsh tribes are beginning to merge into something resembling a nation, through the usual combination of warfare and marriage between the ruling families. Rhodri Mawr (mawr meaning 'the Great') is widely accepted as king of almost the entire region by the time of his death in 878. But centralized power is dissipated by the Celtic custom of sharing an inheritance between all the sons of a royal house.

Rulers in Wales are also, like their neighbours in Anglo-Saxon England, under constant threat from Viking invasions. The Welsh are particularly vulnerable from 838, when the Vikings settle across the Irish Sea in Dublin. But it is Vikings in another form, as Normans, who have a lasting effect in Wales.