The Battle of Maserfield (or Maserfeld, or Maes Cogwy in Welsh) opposed the Anglo-Saxon kings Oswald of Northumbria and Penda of Mercia on August 5, 642. Oswald died in the battle and his kingdom was conquered.
The Welsh annals report 644 as the date, but 642 is the most commonly accepted.
Since the death of Oswald's uncle, Edwin of Northumbria, at Hatfield Chase in 633, the Mercians of Penda have been an obstacle to Northumbrian power in the southern territories.
Oswald won the Battle of Heavenfield in 634 against the Welsh of Cadwallon ap Cadfan (Penda's ally in Hatfield Chase), and thereby re-established Northumbrian hegemony over Great Britain.
Although Penda is said to have recognized Oswald's authority, he may have remained hostile to Northumbria or at least perceived as a threat by Oswald.
The battle
The reasons that led to the Battle of Maserfield are not known. The historian Bede a century later described Oswald as a holy figure. His desire to report the facts in an embellished manner might have caused him to forget to mention Oswald's aggressive warlike tendencies. It only indicates that Oswald died in battle to defend his kingdom.
The location of this battle is traditionally identified as Owestry. If this location is correct, it would mean that Oswald was in enemy territory, and therefore probably in offensive mode.
The result of the battle was the defeat of Northumbria. Bede reports that Oswald prayed for the souls of his soldiers when he felt he was about to die.
Oswald's body was cut to pieces, his head and arms mounted on pikes. The remains were found the following year by his brother and successor Oswiu. As Penda was a pagan and Oswald a Christian, the latter was considered a martyr and a saint. Bede reports a number of miracles around the bones of Oswald and the place where he was killed.
It seems that Eowa, brother of Penda, also king of Mercia, was killed during this same battle. It could be that Eowa has allied with Oswald. It has been suggested that Eowa was the dominant king of the Mercians, controlling the northern territories of Mercia, while Penda would have controlled the southern territories. Since the sources (Historia Brittonum) indicate that Penda would have reigned for 10 years (Bède indicating 22 years, from 633 to 655), this could indicate that his reign is dated as beginning with the victory of the battle of Maserfield, which only makes sense if Eowa's death removed an important rival to Penda, allowing him to consolidate his supremacy over Mercia.
According to Stenton, the Battle of Maserfield gave Penda the status of "the best king in England". He kept this status until his death in 655 during the Battle of Winwaed.