Battle of Ellandun The Battle of Ellandun was fought between Egbert 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 . Background . During the 8th century and early 9th centuries , the kings of Mercia exercised a fluctuating hegemony over the kingdoms of south - eastern England , imposing their overlordship and at times exercising direct rule . While Wessex had at times been obliged to recognise the overlordship of Æthelbald of Mercia , it appears to have escaped the dominance of the Mercians thereafter and was certainly not subject to the more intrusive forms of Mercian control imposed on other kingdoms . Ecgberht ' s father Ealhmund had been king of Kent in the 780s , which had brought his family into conflict with the ambitions of Offa of Mercia , who sought to impose direct rule on Kent . After his father ' s death , Ecgberht was driven from England into exile by Offa , with the co - operation of Beorhtric of Wessex and his accession to the throne of Wessex on Beorhtric ' s death in 802 was immediately followed by a violent clash with Mercia . However , the available sources reveal no further conflict between the two kingdoms before 825 . Beornwulf seized the Mercian throne from Ceolwulf , who had in 821 succeeded on the death of his brother , the long - reigning and powerful Coenwulf . His attack on Wessex two years later may have been part of an effort to consolidate his own authority and reassert that of Mercia after this upheaval . Beornwulf may also have been seeking to take advantage of Ecgberht ' s preoccupation with warfare against the Britons of Cornwall . Ecgberht had devastated Cornish territory in 815 and in the autumn of 825 he was again campaigning against the Britons , at Gafulford . Location . The Battle of Ellandun is thought to have taken place south of Swindon , in Wiltshire , but the exact site has not been discovered . Charles Oman used geographical information and contemporary boundaries as evidence to suggest the battle occurred at Wroughton , which is south of Swindon . T . Spicer has suggested the battle took place on the grounds of what is now Lydiard Park , in Swindon . Consequences . 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 Surrey , 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 . Meanwhile , Beornwulf ' s defeat emboldened the East Angles to revolt against Mercian rule and reassert their independence , in alliance with Wessex . Beornwulf fought the East Angles , but was defeated and killed . His successor Ludeca met the same fate the following year and East Anglian independence was successfully re - established . Ecgberht ' s power peaked in 829 , when he occupied Mercia and secured recognition of his supremacy by the Northumbrians , making him temporarily the overlord of all England . Mercian independence was restored in the following year by Wiglaf , who achieved a significant recovery of Mercian prestige and was even able to extend his power over Berkshire . The independence of East Anglia and the West Saxon conquest of the south - east proved irreversible and Mercia never regained the primacy it had enjoyed in the century before Ellandun .