The period had now arrived when the sovereignty of the Britons was so powerfully disputed that they were compelled to yield to the cohort strength of the impetuous Offa, King of the Mercians. Mercia was the largest of the Anglo-Saxon kingdoms, and London was its capital. Offa passed the Severn with a mighty force, drove the Britons from their fertile and lovely plains, and limited the princedom of Powys to the western side of the celebrated ditch still known by the name of _Offa’s Dyke_. Offa enjoyed a victorious reign, from the year 755 to 794. During that period the finest part of Powys became a confirmed part of the Mercian territory, and Shropshire was permanently annexed to England. Owen and Blakeway, in their invaluable “_History of Shrewsbury_,” remark, “Though there can be no doubt that the cession of Shropshire was obtained from the British Prince (_Eliseg_, it is supposed,) only by the military preponderance of the Saxon, yet it seems equally certain that it must finally have been the subject of a pacific negociation. A work of so much labour as Offa’s Dyke, evidently designed, according to his practice in other places, as the line of demarkation between two kingdoms, could never have been carried into execution without the concurrence of the sovereign on each side of that boundary. * * * * The prince, thus despoiled of the fairest portion of his dominions, retired to Mathrafal, on the Vyrnwy, five miles beyond Welshpool, while Pengwern, degraded from the dignity of a metropolis, passed under the yoke of an English conqueror, and henceforth to be known by the name of Shrewsbury, a name of Saxon origin.”