Under the long reign of Offa, which went on from 758 to 796, it rose again to all but its old dominion.
Since the dissolution of the temporary alliance which Penda formed with the Welsh King Cadwallon the war with the Britons in the west had been the one great hindrance to the progress of Mercia.

But under Offa Mercia braced herself to the completion of her British conquests.

Pushing after 779 over the Severn, and carrying his ravages into the heart of Wales, Offa drove the King of Powys from his capital, which changed its old name of Pengwern for the significant English title of the Town in the Scrub or Bush, Scrobbesbyryg, Shrewsbury.

Experience however had taught the Mercians the worthlessness of raids like these and Offa resolved to create a military border by planting a settlement of Englishmen between the Severn, which had till then served as the western boundary of the English race, and the huge "Offa's Dyke" which he drew from the mouth of Wye to that of Dee.

Here, as in the later conquests of the West-Saxons, the old plan of extermination was definitely abandoned and the Welsh who chose to remain dwelled undisturbed among their English conquerors.