Accession and Overlordship
In 655 Penda besieged Oswiu of Northumbria at Iudeu, the location of which is unknown but which may have been Stirling, in Scotland. Penda took Oswiu's son, Ecgfrith, as hostage, and Oswiu paid tribute, in the form of treasure, to secure Penda's departure. On the way back to Mercia, Oswiu overtook Penda and on 15 November 655 Oswiu and Penda fought on the banks of the (unidentified) river Winwaed. Penda was killed and beheaded by Oswiu, who divided Mercia into northern and southern halves. The northern portion was kept under direct Northumbrian control; the southern kingdom was given to Penda's son Peada, who had married Oswiu's daughter Ealhflæd ca 653.
Peada did not remain king long. He was murdered at Easter in 656, perhaps with the connivance of his wife, Oswiu's daughter. Oswiu then ruled all Mercia himself. Bede lists Oswiu as the seventh and last king to hold imperium (or bretwalda in the language of the Anglo-Saxon Chronicle) over the other Anglo-Saxon kingdoms. Overlordship was a common relationship between kingdoms at this time, often taking the form of a lesser king under the domination of a stronger one. Oswiu went further than this, however, and installed his own governors in Mercia after the deaths of Penda and Peada. This attempt to establish close control of Mercia failed in 658 when three Mercian leaders, Immin, Eafa and Eadbert, rebelled against the Northumbrians. Bede reports that they had kept Wulfhere in hiding, and when the revolt succeeded Wulfhere became king. It has been suggested that the Mercian revolt succeeded because Oswiu may have been occupied with fighting in Pictland, in northern Britain. His nephew the Pictish king Talorgan, son of Eanfrith, had died in 657.
How much direct control Oswiu exerted over the southern kingdoms during his imperium is unclear. Bede describes Oswiu's friendship and influence over Sigeberht of the East Saxons, but generally the pattern in the southeast is of more local domination, with Oswiu's influence unlikely to have been particularly strong. Wulfhere appears to have taken over Oswiu's position in many instances. Bede does not list him as one of the rulers who exercised imperium, but modern historians consider that the rise to primacy of the kingdom of Mercia began in his reign. He seems to have been the effective overlord of Britain south of the Humber from the early 660s, though not overlord of Northumbria as his father had been.
A document called the Tribal Hidage may date from Wulfhere's reign. Drawn up before many smaller groups of peoples were absorbed into the larger kingdoms, such as Mercia, it records the peoples of Anglo-Saxon England, along with an assessment in hides, a unit of land. The Tribal Hidage is difficult to date precisely; it may have been written down in Wulfhere's reign, but other suggested origins include the reign of Offa of Mercia, or Edwin or Oswiu of Northumbria.
Read more about this topic: Wulfhere Of Mercia