Excerpts from Books and Wikipedia
"The Battle of Ellendun or Battle of Wroughton was fought between Ecgberht of Wessex and Beornwulf of Mercia in September 825. Sir Frank Stenton described it as "one of the most decisive battles of English history". It effectively ended Mercian Supremacy over the southern kingdoms of Anglo-Saxon England and established West Saxon dominance in southern England. . . . Ecgberht's victory permanently transformed the political situation in south-eastern England. The king at once sent his son Æthelwulf with an army into the south-east. The West Saxons succeeded in conquering Sussex (hitherto under direct Mercian rule), Kent, and Essex, which had been governed by sub-kings under Mercian overlordship. All of these territories were annexed to Wessex, roughly doubling the kingdom's size." [Wikipedia]