In the eighth century, Mercia was the most powerful kingdom in southern England, but in the early ninth century Wessex became dominant. In the 820s King Egbert of Wessex conquered south-east England (Kent, Surrey, Sussex and Essex). His reign saw the beginning of Viking attacks, but Egbert and his son Æthelwulf, who succeeded in 839, were able to resist them. Æthelwulf died in 858, and he was followed by four sons in succession. King Æthelbald died in 860, and King Æthelberht in 865; Æthelwold's father, Æthelred, then succeeded to the throne. In the same year the Viking Great Heathen Army invaded England. Within five years they had conquered Northumbria and East Anglia, and forced Mercia to buy them off. In late 870 the Vikings invaded Wessex, and in early 871 they fought armies under Æthelred and Alfred in four battles in quick succession, the last two of which Wessex lost. Æthelred died shortly after Easter that year, leaving young sons. Primogeniture was not established in this period, and it was believed that kings should be adults, so he was succeeded by his younger brother Alfred.