Question: The Battle of Aclea occurred in 851 between the West Saxons led by Æthelwulf, King of Wessex from 839 to what year, and the Danish Vikings?

Context paragraphs:
Battle of Bensington: The Battle of Bensington was a major battle fought between Mercia, led by King Offa, and the West Saxons led by Cynewulf of Wessex.  It ended with a victory for the Mercians, and the West Saxons recognizing Mercian overlordship.

Battle of Aclea: The Battle of Aclea occurred in 851 between the West Saxons led by Æthelwulf, King of Wessex and the Danish Vikings.  It resulted in a West Saxon victory.

Cynegils: Cynegils ] was King of Wessex from c. 611 to c. 642.  Cynegils is traditionally considered to have been King of Wessex, but the familiar kingdoms of the so-called Heptarchy had not yet formed from the patchwork of smaller kingdoms in his lifetime.  The later kingdom of Wessex was centred on the counties of Hampshire, Dorset, Somerset, and Wiltshire, but the evidence of the "Anglo-Saxon Chronicle" is that the kingdom of Cynegils was located on the upper River Thames, extending into northern Wiltshire and Somerset, southern Gloucestershire and Oxfordshire, and western Berkshire, with Dorchester-on-Thames as one of the major royal sites.  This region, probably connected to the early tribal grouping known as the Gewisse, a term used by Bede for the West Saxons, lay on the frontier between the later kingdoms of Wessex and Mercia.

Æthelstan of Kent: Æthelstan (died c. 852), the eldest son of King Æthelwulf of Wessex, was the King of Kent from 839 under the authority of his father.  The late D, E and F versions of the "Anglo-Saxon Chronicle" describe Æthelstan as Æthelwulf's brother, but the A, B and C versions, and Æthelweard's "Chronicon", state that he was Æthelwulf's son.  Some historians have argued that it is more probable that he was a brother, including Eric John in 1966 and Ann Williams in 1978.  However, in 1991 Ann Williams described him as Æthelwulf's son, and this is now generally accepted by historians, including Frank Stenton, Barbara Yorke, and D. P. Kirby.

Battle of Hingston Down: The Battle of Hingston Down took place in 838 at Hingston Down in Cornwall between a combined force of Cornish and Vikings on the one side, and West Saxons led by Egbert, King of Wessex on the other.  It resulted in a West Saxon victory.

Æthelberht, King of Wessex: Æthelberht (or Ethelbert; Old English: "Æþelberht" , meaning "magnificent noble") was the King of Kent from 858 and of Wessex from 860 until his death in 865.  He was the third son of Æthelwulf of Wessex and his first wife, Osburh.  In 855 he became under-king of Kent while his father, Æthelwulf, visited Rome.  His brother Æthelbald was left in charge of the West Saxons.  After his father's death in 858 he succeeded him as king of Kent and the other eastern parts of the kingdom.  When Æthelbald died childless in 860, the kingship of the West Saxons also passed to Æthelberht.

Beorhtwulf of Mercia: Beorhtwulf (] , meaning "bright wolf"; also spelled "Berhtwulf"; died 852) was King of Mercia, a kingdom of Anglo-Saxon England, from 839 or 840 to 852.  His ancestry is unknown, though he may have been connected to Beornwulf, who ruled Mercia in the 820s.  Almost no coins were issued by Beorhtwulf's predecessor, Wiglaf, but a Mercian coinage was restarted by Beorhtwulf early in his reign, initially with strong similarities to the coins of Æthelwulf of Wessex, and later with independent designs.  The Vikings attacked within a year or two of Beorhtwulf's accession: the province of Lindsey was raided in 841, and London, a key centre of Mercian commerce, was attacked the following year.  Another Viking assault on London in 851 "put Beorhtwulf to flight", according to the "Anglo-Saxon Chronicle"; the Vikings were subsequently defeated by Æthelwulf.  This raid may have had a significant economic impact on Mercia, as London coinage is much reduced after 851.

Æthelwulf: Æthelwulf (Old English for "Noble Wolf"; died 13 January 858) was King of Wessex from 839 to 858.  In 825, his father, King Egbert, defeated King Beornwulf of Mercia, ending a long Mercian dominance over Anglo-Saxon England south of the Humber.  Egbert sent Æthelwulf with an army to Kent, where he expelled the Mercian sub-king and was himself appointed sub-king.  After 830, Egbert 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.

Wulfhere of Mercia: Wulfhere or Wulfar (died 675) was King of Mercia from 658 until 675 AD.  He was the first Christian king of all of Mercia, though it is not known when or how he converted from Anglo-Saxon paganism.  His accession marked the end of Oswiu of Northumbria's overlordship of southern England, and Wulfhere extended his influence over much of that region.  His campaigns against the West Saxons led to Mercian control of much of the Thames valley.  He conquered the Isle of Wight and the Meon valley and gave them to King Æthelwealh of the South Saxons.  He also had influence in Surrey, Essex, and Kent.  He married Eormenhild, the daughter of King Eorcenberht of Kent.

Eohric of East Anglia: Eohric (died 902) was a Danish Viking king of East Anglia.  The name "Eohric" is the Old English form of the Old Norse "Eiríkr".  Little is known of Eohric or of the kingdom of East Anglia in his time.  The "Anglo-Saxon Chronicle" recorded that an army from East Anglia raided Mercia and Wessex, and a West Saxon army raided East Anglia in retaliation.  The Vikings met a section of the West Saxons at the Battle of the Holme on 13 December 902, and Eohric was killed.

Supporting paragraphs: Battle of Aclea Æthelwulf
Answer: 858