Its centre was in the valley of the River Trent, and its tributaries, but later it grew to extend from the Humber to the Thames, and west to the borders of Wales.
According to the Saxon Chronicle, the Mercians came originally from the Anglian region of Germany, along with the East and Middle Angles and other peoples who colonised the lands north of the Humber. Their coming-over is not dated but an entry for 655CE records that the "Mercians" became Christian, so they must have been in England in sufficient numbers by then to constitute a seprate people.
On this basis we can say that the kingdom roughly shook itself into shape around the 6th and 7th centuries, although 655CE is also the year in which King Penda is supposed to have been killed, succeeded by his son Peada, who was killed next year "betrayed by his queen". Still, by 675, and the death of Wufhere, Penda's son, the Mercians had a minster, at Medhamsted, and were on at least their 3rd bishop.
Mercia's first major line of kings were the descendants of Penda. In their wake came a line claiming descent from one Eowa, which included the Offa of Offa's Dyke, an early border marker between the Anglo-Saxon lands and those of the Welsh princes. The other notable member of this line appears to have been Ethelbald, who ruled for 41 years (716-755), being succeeded by Bernred, whom Offa "put to flight".
Offa tried to secure his sucession by murdering many of the candidates. In the event this handed the kingdom, after the death of Offa's son Egfrith, to a rival line, that of Coenwalh. Within half a century, after attacks on the Kentish and East Anglians kingdoms, Mercia became a client kingdom of Wessex, initially for about a year in 827-8CE. Later, in 853, the kingdoms allied to "conquer" North Wales, and the Mercian king married the daughter of Ethelwulf, King of the West Saxons.
In 868 a "heathen" army ravaged Mercia, till they were bought off. That lasted till 870, when the heathens returned. The Mercians paid up again in 872 and 873; in 874 their king was killed and his successor, Ceolwulf II, swore to the heathen service. This, however, did little or no good: in 877 the kingdom was raided again, and after an Aethelred (879-884) the line of kings was reduced to one of client eorldermen under Wessex, and in 919CE the kingdoms were merged.
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