c.626. Penda becomes king of the Mercians
Sources for the early history of Mercia are few and far between, with almost no detail before we reach Penda in the early 7th century. The earlier material can be dealt with briefly here. The royal line is said to go back, son to father, from Penda to Pypba to Crida to Cynewald to Cnebba to Icel. Icel is probably the one who came to Britain, since the 8th-century Life of St Guthlac notes that the line begins with him. The Anglo-Saxon Chronicle (ASC 626) takes the line farther back, through Eomer and Angeltheow and Offa (three legendary heroes noted in Beowulf), and eventually to Woden. This neat structure ignores the Mercian king Cearl and his daughter Cyneburh, who took in the Northumbrian Edwin in his exile according to Bede (HE, ii.14), and so presumably ruled at some point between the end of the 6th century and 616. Henry of Huntingdon in the 12th century assumed Cearl was a kinsman of Pypba and reigned between Pypba and Penda, but this may be no more than a guess (see Sims-Williams, p.25).
Penda probably started to rule in about 626. The Anglo-Saxon Chronicle notes under 626 that he ruled the kingdom for 30 years, and was 50 when he gained the throne. This would make him of Lear-like proportions at his final battle in 655, and it is more likely that the Chronicle is mangling a source which said that he ruled the kingdom for 30 years and was 50 when he died (see Brooks, p.166). Bede notes that Penda held the kingdom of the Mercians for 22 years after the battle of Hatfield in 633, "with varying success" (HE, ii.20); this might be taken to imply that Penda came to power in 633, but it might only mean that he first came to Bede's Northumbrian attention in the year of his major battle with the Northumbrians. (The West Saxons would remember Penda from their first battle with him in 628, recorded in the Chronicle.) The Historia Brittonum is more difficult to reconcile, since it gives Penda a reign of ten years, starting from the battle of Maserfelth (in 642), at which his brother Eowa, king of the Mercians, was killed, and by means of which he freed the kingdom of the Mercians from the Northumbrians. This starting date of 642, at odds with the other two sources, suggests that Bede's brief comment that Penda ruled "with varying success" may conceal a period when Penda had lost control of Mercia to the Northumbrians (see entry on 634-42).
Penda's relations with the Northumbrians, then, are a tale of four battles: first, the battle of Hatfield in 633, in which he and Cadwallon of Gwynedd killed Edwin of Northumbria, and second, in the following year, the battle at Denisesburn (see entries on 634 and 634-42), in which Cadwallon at least was soundly defeated by Oswald of Northumbria, and Penda may have had his brother imposed on him as king of the Mercians under the overlordship of the Mercians. Penda would defeat the Northumbrians a second time in 642 at the battle of Maserfelth, when he would kill Oswald of Northumbria, and Penda would himself be killed in the Northumbrians' second victory, at the battle of Winwæd of 655.
Penda's relations with other southern kingdoms were scarcely more peaceful. He defeated Cynegils and Cwichelm of Wessex in 628 at Cirencester, probably establishing his overlordship over the Hwicce at that point, and possibly forcing Cynegils's son Cenwealh to marry his sister. When Cenwealh repudiated Penda's sister in about 645, Penda forced him into exile, and he fled to the court of Anna of the East Angles. The East Angles were likely to harbour Penda's enemies because he had killed two of their kings, Sigeberht and Ecgberht, in a battle of c.635?645; he would go on to kill Anna of the East Angles in 654. It may be that one of the things that Penda was disputing with the East Angles was jurisdiction over the territory of the Middle Angles which lay between them, over which Penda put his son Peada in 653. It may also be that Penda was systematically reducing the power of a kingdom which had flexed its muscles and shown that it might be a threat back in 616 when it had sent an army up to the Northumbrian border (presumably through eastern Mercia) and toppled the Northumbrian king (see Dumville, p.132).
N. Brooks, "The formation of the Mercian kingdom", The Origins of the Anglo-Saxon Kingdoms (London: 1989), pp.159-70
D. Dumville, "Essex, Middle Anglia and the expansion of Mercia", The Origins of the Anglo-Saxon Kingdoms (London: 1989), pp.123-40
P. Sims-Williams, Religion and Literature in Western England, 600-800 (Cambridge: 1990)