Following a failed attack at the Dane encampment at Readingum, Æthelred I and his brother Alfred desired to draw out the Danes from their fortifications to an open field battle in Ashdown. After intercepting a message that relayed that information to Burgred of Mercia, jarl Sidroc the Elder, who had been marching with his men to meet Guthrum and Halfdan Ragnarsson at Readingum, decided to march to Ashdown.
Upon their arrival, Geirmund Hel-hide noticed the Saxons were stationed atop the north mound, the Danes atop the south mound and an empty open field between the armies, with neither side giving up the advantage of a high ground to attack their enemy. Sidroc and his group crossed the river near the north mound, attempting to flank, though the Saxons divided their forces while they crossed, sending half of them to battle the group while the rest stayed atop the dun.
Halfdan's forces also separated, with Halfdan going around to attempt a sneak attack while Sten Stensson and his sons Ulf and Björn drew the Saxons' attention by attacking from the front, though the Saxons moved to prevent his army from joining Sidroc's.
Spotting Æthelred's coat of arms fleeing from the scene, the Stenssons moved towards it, killing the Saxons standing in their way, only to have it be a trick by Ælfred. Ælfred fired an arrow against Björn, but he was saved by Sten who took the blow instead. Ælfred escaped from the scene as another Saxon clan, led by Æthelred, arrived. Björn and Ulf took their father and carved their way out of the battle.
The Saxons managed to defeat the forces of Sidroc, which they outnumbered, and the remaining warriors fled with the Saxons in pursuit. While most fled north, some, like Geirmund, went south, to join Halfdan's army.
At the end of the day, even with many killed, the Saxons conquered the field. The Danes suffered heavy losses, among them, King Bersi, jarl Sidroc the Elder, his son Sidroc, jarl Osbern, as well as jarl Stensson.
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