Mercia, that part of Saxon England which bordered on the Welsh Marches; hence its name. The kingdom was founded in the 6th century, and at first covered little more than Staffordshire. Penda, its first great king, not only changed it from a mass of independent settlements into one great kingdom, but even for a time checked the progress of Christianity. His nephew, Wulfhere, reestablished the position of Mercia, which had been lost at Penda's death, but submitted to Christianity. During the 8th century under AEthelbald, Offa, and Cenwulf, it was the most powerful of the Saxon kingdoms; but after the battle of Ellandun (823) it finally succumbed to Wessex, though there was a revolt under Edwy, and a kind of semi-independence followed its establishment by Cnut as one of the four earldoms.