oswiu invaded deira in 651 seeking to reunite deira with bernicia. oswine was betrayed by one of his own noblemen and was murdered in 651 on oswiu's orders. but the deirans chose athelwald, oswiu's nephew, as their king. athelwald then aligned himself with oswiu's enemy, penda. deira was under the protection of mercia for the next three years. but penda was determined to defeat oswiu and take bernicia as well. in 654 penda raised a large army of some thirty 'legions'. according to bede, penda attacked oswiu several times. with a much smaller force oswiu attacked penda's army near the banks of the river winwaed. most of the commanders of the thirty legions were killed including king anna of east anglia and penda. from this time oswiu became the or overlord over all the southern english people including mercia. mercia was then divided. north of the river trent was controlled directly by oswiu. he made peada of mercia, penda's son, king over the part of mercia south of the trent. peada had married oswiu's daughter, alflaed. peada was murdered five months later by alflaed, possibly on oswiu's orders. mercians rebelled against oswiu and peada's brother wulfhere became king of mercia.