During the Heptarchy, the most powerful king among the Anglo-Saxon kingdoms might become acknowledged as Bretwalda, a high king over the other kings. The decline of Mercia allowed Wessex to become more powerful. It absorbed the kingdoms of Kent and Sussex in 825. The kings of Wessex became increasingly dominant over the other kingdoms of England during the 9th century. In 827, Northumbria submitted to Egbert of Wessex at Dore, briefly making Egbert the first king to reign over a united England.