In 794 a pirate band plundered the monasteries of Wearmouth and Jarrow, and the presence of the freebooters soon told on the political balance of the English realms. A great revolution was going on in the south, where Mercia was torn by civil wars which followed on Cenwulf's death, while the civil strife of the West-Saxons was hushed by a new king, Ecgberht. In Offa's days Ecgberht had failed in his claim of the crown of Wessex and had been driven to fly for refuge to the court of the Franks. He remained there through the memorable year during which Charles the Great restored the Empire of the West, and returned in 802 to be quietly welcomed as King by the West-Saxon people. A march into the heart of Cornwall and the conquest of this last fragment of the British kingdom in the south-west freed his hands for a strife with Mercia, which broke out in 825 when the Mercian King Beornwulf marched into the heart of Wiltshire. A victory of Ecgberht at Ellandun gave all England south of Thames to the West-Saxons, and the defeat of Beornwulf spurred the men of East-Anglia to rise in a desperate revolt against Mercia. Two great overthrows at their hands had already spent its strength when Ecgberht crossed the Thames in 828, and the realm of Penda and Offa bowed without a struggle to its conqueror. But Ecgberht had wider aims than those of supremacy over Mercia alone. The dream of a union of all England drew him to the north. Northumbria was still strong; in learning and arts it stood at the head of the English race; and under a king like Eadberht it would have withstood Ecgberht as resolutely as it had withstood AEthelbald. But the ruin of Jarrow and Wearmouth had cast on it a spell of terror. Torn by civil strife, and desperate of finding in itself the union needed to meet the northmen, Northumbria sought union and deliverance in subjection to a foreign master. Its thegns met Ecgberht in Derbyshire, and owned the supremacy of Wessex. [Sidenote: Conquests of the Northmen]