Mercia was the dominant kingdom in southern England in the eighth century, and maintained its position until it suffered a decisive defeat by King Egbert of Wessex at the Battle of Ellendun in 825. Egbert briefly conquered Mercia, but it recovered its independence in 830, and thereafter the two kingdoms became allies, which was to be an important factor in English resistance to the Vikings. The Mercians traditionally held overlordship over Wales, and in 853 King Burgred of Mercia obtained the assistance of King Æthelwulf of Wessex in an invasion of Wales in order to reassert their hegemony. The same year, Burgred married Æthelwulf's daughter.