The battle of Aclea was fought in the year 851 by Aethelwulf, king of Wessex against an invading force of Viking insurgents.
All that we know of this battle is derived from the Anglo-Saxon Chronicle which for the year 851 recorded that;
three-and-a-half hundred ships came into the mouth of the Thames, and stormed Canterbury and London and put to flight Beorhtwulf king of Mercia with his army, and then went south over the Thames into Surrey: and king Aethelwulf and his son Aethelbald with the West Saxon army fought against them at Oak Field (Aclea), and there made the greatest slaughter of a heathen raiding-army that we have heard tell of up to the present day, and there took the victory.
(The "present day" was of course, some forty years later when work on the Chronicle first began.)
The battle of Aclea is therefore notable as one of the few decisive and overwhelming victories of the English over the Vikings during the first phase of Viking insurgency in the period 793 to 865; the arrival of the Viking 'Great Army' in 865 heralded an intensification in the conflict that made the victory of 851 somewhat academic.
The exact location of the battle has never been identified. Although the Old English 'Aclea' is probably the modern Oakley there a number of settlements with that name. Suggested locations include the Oakley that lies some four miles to the north west of Bedford on the Great Ouse and the village of Water Oakley in Berkshire.
SOURCES
The Winchester manuscript of the Anglo-Saxon Chronicle from the translation by Michael SwantonThe Anglo-Saxon Chronicles (Phoenix Press, 2000)
Ann Williams, Alfred P. Smyth and D. P. Kirby A Biographical Dictionary of Dark Age Britain (Seaby, 1991)