There "he made a terrible slaughter of the perfidious race." Over two thousand Welsh monks from the monastery of Bangor Iscoed were slain by the heathen invader; but Baeda explains that AEthelfrith put them to death because they prayed against him; a sentence which strongly suggests the idea that the English did not usually kill non-combatant Welshmen.
The victory of Chester divided the Welsh power in the north as that of Deorham had divided it in the south.

Henceforward, the Northumbrians bore rule from sea to sea, from the mouth of the Humber to the mouths of the Mersey and the Dee.

AEthelfrith even kept up a rude navy in the Irish Sea.

Thus the Welsh nationality was broken up into three separate and weak divisions--Strathclyde in the north, Wales in the centre, and Damnonia, or Cornwall, in the south.

Against these three fragments the English presented an unbroken and aggressive front, Northumbria standing over against Strathclyde, Mercia steadily pushing its way along the upper valley of the Severn against North Wales, and Wessex advancing in the south against South Wales and the West Welsh of Somerset, Devon, and Cornwall.