Battle of the Winwaed The Battle of the Winwaed ( Brythonic : " Maes Gai " ; ) was fought on 15 November 655 ( or 654 ) between King Penda of Mercia and Oswiu of Bernicia , ending in the Mercians ' defeat and Penda ' s death . History . Although the battle is said to be the most important between the early northern and southern divisions of the Anglo - Saxons in Britain , few details are available . The two armies met on the banks of a river named the " Winwaed " , but this river has never been identified . Possibly it was a tributary of the Humber . There is reason to believe it may have been the river now known as Cock Beck in the ancient kingdom of Elmet , which passes Pendas Fields , Leeds , before joining the River Wharfe ( which eventually feeds into the Humber ) . Another possibility is the River Went , a tributary of the River Don , situated to the north of modern - day Doncaster . It could also be in Oswestry or Winwick . The roots of the battle lay in Penda ' s success in dominating England through a number of military victories , most significantly over the previously dominant Northumbrians . In alliance with Cadwallon ap Cadfan of Gwynedd he had defeated and killed Edwin of Northumbria at Hatfield Chase in 633 , and subsequently he defeated and killed Oswald of Northumbria at the Battle of Maserfield in 642 . Maserfield effectively marked the overthrow of Northumbrian supremacy , and in the years that followed the Mercians apparently campaigned into Bernicia , besieging Bamburgh at one point ; the Northumbrian sub - kingdom of Deira supported Penda during his 655 invasion . Battle . Penda , after gathering allies from East Anglia and Wales , marched with a force led by " thirty warlords " . Oswiu , who was Oswald ' s brother but had succeeded him only in Bernicia , the northern part of Northumbria , was besieged by Penda ' s forces at a place called " Iudeu " ( identified with Stirling ) in the north of his kingdom . Apparently Oswiu was desperate enough to offer a great deal of treasure to Penda in exchange for peace . Although the sources are unclear , it is likely that some sort of agreement was reached at " Iudeu " : although Bede says that Oswiu ' s offers of treasure were rejected by Penda , who , Bede says , was determined to destroy Oswiu ' s people " from the highest to the lowest " , he does mention that Oswiu ' s young son Ecgfrith was being held hostage by the Mercians , perhaps as part of a deal . The " Historia Brittonum " contradicts Bede regarding the treasure , saying that Penda distributed it among his British allies , which would presumably mean that he accepted it . The recorded events may be interpreted to mean that Penda and his army then began marching home , but for some reason the two armies met and fought at a place called the River " Winwaed " . Breeze ( 2004 ) argues that Penda and his army would have been in a difficult strategic location along the Went during their withdrawal , giving Oswiu a good opportunity to attack . It is almost certain that the small ( " perparvus " , according to Bede ) Northumbrian forces were considerably outnumbered by the Mercians and their allies . According to Bede , before the battle Oswiu prayed to God and promised to make his daughter a nun and grant twelve estates for the construction of monasteries if he was victorious . Penda ' s army was apparently weakened by desertions . According to the " Historia Brittonum " , Penda ' s ally Cadafael ap Cynfeddw of Gwynedd ( thereafter remembered as " Cadomedd " ( = " battle - shirker " ) ) abandoned him , along with his army , and Bede says that Aethelwald of Deira withdrew from the battle to await the outcome from a place of safety . Penda was soundly defeated , and both he and his ally , the East Anglian King Aethelhere , were killed , with thirty allied leaders of warbands ( " duces regii " ) . The battle was fought by the river in the midst of heavy rains , and Bede says that " many more were drowned in the flight than destroyed by the sword " . Bede mentions that Penda ' s head was cut off . Writing in the 12th century , Henry of Huntingdon expanded his version of Bede ' s text to include supernatural intervention and remarked that Penda , in dying violently on the battlefield , was suffering the same fate he had inflicted on others during his aggressive reign . Aftermath . The battle had a substantial effect on the relative positions of Northumbria and Mercia . Mercia ' s position of dominance , established after the battle of Maserfield , was destroyed , and Northumbrian dominance was restored ; Mercia itself was divided , with the northern part being taken by Oswiu outright and the southern part going to Penda ' s Christian son Peada , who had married into the Bernician royal line ( although Peada survived only until his murder in 656 ) . Northumbrian authority over Mercia was overthrown within a few years , however . Significantly , the battle marked the effective demise of Anglo - Saxon paganism ; Charles Plummer , in 1896 , described it as " decisive as to the religious destiny of the English " . Penda had continued in his traditional paganism despite the widespread conversions of Anglo - Saxon monarchs to Christianity , and a number of Christian kings had suffered death in defeat against him ; after Penda ' s death , Mercia was converted , and all the kings who ruled thereafter ( including Penda ' s sons Peada , Wulfhere and Æthelred ) were Christian .