For nearly 300 years, Mercia was the most powerful of the Anglo-Saxon kingdoms. When Ecgfrith, king of the Northumbrians, was killed in 685, Mercia filled the power vacuum, coming to dominate the land south of the Humber, with only the kingdom of Wessex holding out against Mercian hegemony. But of the three major Anglo-Saxon kingdoms – Northumbria, Wessex and Mercia – the history of Mercia mostly comes from the pens of its enemies. Most notable among these is Bede, a proud Northumbrian, who despite the otherwise broad sweep of his History, treats the Mercians pretty well only as antagonists.
The name Mercia derives from Mierce, an Old English word meaning the ‘marches’ or ‘border people’, and that is what it was when first settled: the border kingdom between the Anglo-Saxon kingdoms of the south and east and the Britonnic kingdoms of the west and north. These people settled in the Midlands, following the river valleys into the heart of the country. The king lists of the Mercians traced their lineage back to Icel, an Anglian prince who settled in Britain, giving the ruling family the name Iclingas.
However, the first king to be reliably recorded is Penda, the great enemy of the kings of Northumbria, who killed two of them (Edwin and Oswald), as well as three kings of East Anglia. Penda was the last great pagan king of the Anglo-Saxons and when he fell in battle with Oswiu, Oswald’s brother and successor as king of Northumbria, the conversion of the Anglo-Saxons to Christianity was guaranteed.
Mercia and Northumbria continued to struggle for dominance until, with the death of King Ecgfrith in 685, Mercian supremacy was assured. It reached its height during the reign of King Offa in 757-96, when Mercian power encompassed the whole country and Offa could deal, almost as an equal, with no less a king than Charlemagne.