Battle of Maserfield
King Oswald's uncle, King Edwin of Northumbria, was killed at the Battle of Hatfield Chase in 633 by Penda and his ally Cadwallon ap Cadfan. After the battle Cadwallon and Penda began slaughtering the people of Northumbria. This included men, women and children. Northumbria fell apart and was divided into its two former kingdoms of Deira and Bernicia. Oswald defeated and killed Cadwallon ap Cadfan at the Battle of Heavenfield in 634. After that Oswald reunited Northumbria into a single kingdom. Oswald was considered a bretwalda by Bede, the fourth such overlord over the other Anglo-Saxon kings. Penda remained a constant threat but was kept busy fighting the East Angles and making new alliances with the Welsh. Oswald also created several similar alliances in an effort to contain Penda. About 638 he won a battle against Owen Map Bili, the king of Strathclyde near Edinburgh. In 640 Oswald recovered the kingdom of Lindsey from Penda.