The westward progress of the Northumbrians is later and much more
historical. Theodoric, son of Ida, as we may perhaps infer from the old
Welsh ballads, fought long and not always successfully with Urien of
Strathclyde. But in 592, says Bæda, who lived himself but three-quarters
of a century later than the event he describes, "there reigned over the
kingdom of the Northumbrians a most brave and ambitious king,
Æthelfrith, who, more than all other nobles of the English, wasted the
race of the Britons; for no one of our kings, no one of our chieftains,
has rendered more of their lands either tributary to or an integral part
of the English territories, whether by subjugating or expatriating the
natives." In 606 Æthelfrith rounded the Peakland, now known as
Derbyshire, and marched from the upper Trent upon the Roman city of
Chester. 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 Bæda explains that Æthelfrith 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. Æthelfrith 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