[Adapted from material at http://en.wikipedia.org/wiki/Northumbria]
It was composed of the independent kingdoms of Bernicia and of Deira. Its first unified king was Athelfrith of Bernicia, who conquered Deira around 604CE. He was killed in battle around 616 by Raedwald of East Anglia, who installed Edwin, son of a former Deiran king, as Northumbrian king. Edwin accepted Christianity in 627CE, and conquered the Isle of Man and the North Welsh kingdom of Gwynedd before being defeated by an alliance of Gwynedd's exiled king, Cadwallon, and Penda, king of Mercia.
At its greatest extent Northumbria reached from the River Humber (which gave it part of its name) to the River Forth in what would later be Scotland. This meant that its borders fluctuated, depending on the relative strengths of the armies of Northumbria and the various English and Scottish kingdoms.
As the kingdom declined, with the strengthening of Scotland closing its northern extent and the English kingdoms unifying to the south, it became an earldom, which then itself became an object of dispute between the English and Scottish kings over whom should control it.
After the Norman Conquest William obtained the alleigiance both of the earl and of the Bishop of Durham. However a revolt ensued and a Norman noble, Robert Comine, who was to have been the next Earl, was massacred. In revenge, William drove into the North of England with troops, and devastated the rebels' lands.
Under his son, William Rufus, Northumbria was divided, the southern section becoming the county palatine of Durham, under the Bishop, the northern (beyond the Tyne) bcecoming Northumberland, with an earl appointed through the English crown, and the Bishop of Durham having his own fiefdoms (such as Hexhamshire, which for a time was also a palatinate area.
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