Æthelbald was King of Wessex from 855 or 858 to 860. He was the second of five sons of King Æthelwulf. In 850, Æthelbald's elder brother Æthelstan defeated the Vikings in the first recorded sea battle in English history, but he is not recorded afterwards and probably died in the early 850s. The next year Æthelwulf and Æthelbald inflicted another defeat on the Vikings at the Battle of Aclea. In 855, Æthelwulf went on pilgrimage to Rome and appointed Æthelbald King of Wessex, while Æthelberht, the next oldest son, became King of Kent, which had been conquered by Wessex thirty years earlier.

Æthelwulf was King of Wessex from 839 to 858. In 825, his father, King Ecgberht, defeated King Beornwulf of Mercia, ending a long Mercian dominance over Anglo-Saxon England south of the Humber. Ecgberht sent Æthelwulf with an army to Kent, where he expelled the Mercian sub-king and was himself appointed sub-king. After 830, Ecgberht maintained good relations with Mercia, and this was continued by Æthelwulf when he became king in 839, the first son to succeed his father as West Saxon king since 641.