The reign of the legendary King Offa of Mercia ended in 796. Although Mercia continued to be a dominant force in England for much of the next century, the kingdom of Wessex began to grow in power around the time of Offa’s death. Wessex achieved important victories against its main rival under the reign of King Egbert between 802–39. Besides Northumbria, which maintained a degree of independence, Mercia held a great deal of power over its rival kingdoms in the south and was able to exert both political and military control over them. To the west of Mercia, the Celtic tribes in Wales proved troublesome enough that a great dyke was built during the reign of Offa to act as a border between the two territories. Around this time the first Viking raids were taking place on the east coast and would soon increase in severity.
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