The Battle of Aclea occurred in 851 between the West Saxons led by Æthelwulf, King of Wessex and the Danish Vikings at an unidentified location in England (noted by near-contemporaries as being in Surrey). It resulted in a West Saxon victory.
Little is known about the battle and the most important source of information comes from the Anglo-Saxon Chronicle which recorded that:
Æthelwulf (Old English: [ˈæðelwuɫf]; Old English for "Noble Wolf"; died 13 January 858) 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.
The Vikings were not a major threat to Wessex during Æthelwulf's reign. In 843, he was defeated in a battle against the Vikings at Carhampton in Somerset, but he achieved a major victory at the Battle of Aclea in 851. In 853, he joined a successful Mercian expedition to Wales to restore the traditional Mercian hegemony, and in the same year, his daughter Æthelswith married King Burgred of Mercia. In 855, Æthelwulf went on a pilgrimage to Rome. In preparation he gave a "decimation", donating a tenth of his personal property to his subjects; he appointed his eldest surviving son Æthelbald to act as King of Wessex in his absence, and his next son Æthelberht to rule Kent and the south-east. Æthelwulf spent a year in Rome, and on his way back he married Judith, the daughter of the West Frankish king Charles the Bald.
Æthelbald (died 860) 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.
On his way back from Rome, Æthelwulf stayed for several months with Charles the Bald, King of the West Franks, whose twelve-year-old daughter Judith he married. When he returned to England in 856, Æthelbald refused to give up the crown. Most historians believe that Æthelbald continued to be king of Wessex while Æthelberht gave up Kent to his father, but some think that Wessex itself was divided, with Æthelbald ruling the west and his father the east, while Æthelberht kept Kent. When Æthelwulf died in 858, Æthelbald continued as (or became again) king of Wessex and his brother resumed (or carried on) his kingship of Kent.