Egbert of Wessex (l. c. 770-839 CE, r. 802-839 CE; also given as Ecgberht, Ecbert) was the most powerful and influential king of Wessex prior to the reign of Alfred the Great (r. 871-899 CE). Egbert came to the throne at a time when the neighboring Kingdom of Mercia had dominated Wessex and controlled the sitting king Beorhtric (786-802 CE) through an alliance sealed by marriage. Egbert seems to have taken time to assemble his military and resources and then met the Mercian armies and defeated them at the Battle of Ellandun in 825 CE. Afterwards, he swiftly took Mercian territory, installed his son Aethelwulf (r. 839-858 CE) as sub-king, and neutralized Mercian aggression. Egbert was the first king of Wessex to completely subdue Mercia and the stability he provided allowed for further development of the kingdom as well as the resources to withstand the Viking raids. At his death, he was so powerful that the Anglo-Saxon Chronicles refer to him as Ruler of Britain, not just King of Wessex.
This is a companion discussion topic for the original content at https://www.worldhistory.org/King_Egbert_of_Wessex/.