Mercia (mûr´shə), one of the kingdoms of Anglo-Saxon England, consisting generally of the region of the Midlands. It was settled by Angles c.500, probably first along the Trent valley. Its history emerges from obscurity with the reign of Penda, who extended his power over Wessex (645) and East Anglia (650) to gain overlordship of England S of the Humber River. After his death Mercia suffered a three-year loss of ascendancy during which it was converted to Christianity by a Northumbrian mission. Penda's son, Wulfhere, then reestablished a Greater Mercia that finally, under Æthelbald in the 8th cent., extended over all S England. This hegemony was strengthened by Offa (reigned 757–96), who controlled East Anglia, Kent, and Sussex and maintained superiority of a sort over Wessex and Northumbria. He had the great Offa's Dyke built to protect W Mercia from the Welsh. After his death, Mercian power gradually gave way before that of Wessex. The victories of Egbert of Wessex in Mercia established him briefly as overlord. In 874, Mercia weakly succumbed to the invading Danish army, and ultimately the eastern part became (886) a portion of the Danelaw, while the western part was controlled by Alfred of Wessex. Thereafter Mercia had no independent history, although it had one more distinguished ruler in Æthelflæd, Lady of the Mercians.
See F. M. Stenton, Anglo-Saxon England (3d ed. 1971).