Penda secures dominance over the area of Middle Anglia, which had been incorporated into the wider kingdom of Mercia, apparently well before the reign of Penda, who evidently feels safe enough to locate his base in the territory of the Middle Angles, over whom he places his eldest son, Peada, as sub-king.

Bede specifies the Middle Angles as the target of a four-man Christian mission accepted by Peada, who converts to Christianity, partly in order to wed Alchflaed, the daughter of King Oswiu of Northumbria.

This mission arrives in 653 and includes St Cedd.

Peada's conversion and acceptance of baptism in Northumbria by Bishop Finan of Lindisfarne possibly indicates a continuing sense of disunity or local particularism within Mercia.

It is unlikely that Peada could have pursued so different a course from his father, at the strategic and political center of the Mercian kingdom, without local support among the Middle Angles.

Œthelwald had become king of Deira in 651 after Oswine of Deira was killed by Oswiu of Bernicia; it is uncertain whether Oswiu (who was Œthelwald's uncle) installed him as king or whether Œthelwald took the kingship in opposition to Oswiu.

Œthelwald, a pious Christian, rejects Oswiu's overlordship and turns instead to the pagan Penda, who mounts another attack on Bernicia.

King Sigeberht I of Essex dies after a thirty-six-year reign and is succeeded by his relative Sigeberht II.

Sigeberht finds a powerful northern "friend" (amicus) and ally in King Oswiu of Bernicia, who persuades him to adopt Christianity as part of a mobilization against Penda.

Bede's statement that Sigeberht regularly visited the Bernician court and the general nature of Oswiu's influence on the course of Sigeberht's career suggest that the balance of power was in Oswiu's favor.

Oswiu may have intended the alliance to help him offer effective resistance against King Penda of Mercia, as is also suggested by his alliance with Penda's son Peada, king of the Middle Angles.