The northern-most of the Anglo-Saxon kingdoms, Northumbria’s southern border ran along the river Trent, a bit south of York, and went straight across the island. Its lands stretched north to Edinburgh, but did not include large chunks of the west, which were still held by the British Kingdoms of Rheged and Clyde. Northumbria became a permanent, independent state under the rule of Aethelferth. It would also be lead by the notable Kings: Edwin, Oswald, and Oswy.
Mercia was the largest of the Anglo-Saxon kingdoms. It northern border was the same as the southern border of Northumbria. In the west, it bordered the British Kingdoms of modern Wales. To the south, the kingdom ended at the River Thames and looked south upon the West Saxons (Wessex). It also shared borders with the smaller Kingdoms of the East Saxons (Essex) and East Anglia.
Although the largest, Mercia was not as unified as some of the other kingdoms in the early years of the 7th century. It would begin its rise to power under Penda, the last great pagan king of the Saxons. Soon afterward, it would take its place as the most powerful of the Saxon Kingdoms and hold this dominance until the Danish invasions. Mercia’s most famous king was Offa, who constructed the great Dyke along the border of Wales.
Wessex (which is also know at this time as the Kingdom of the West Saxons) sat just below Mercia, between the British Kingdom of Dumnonia on the Cornish Peninsula and stretched east nearly to London. During the 7th to 9th century, Wessex was rarely a unified state, and was ussually under the control of a number of under-kings, though occasionally a strong leader would unify them. Wessex would truly come to the fore during the Danish Invasions when the Mercian Kings (most notably Alfred the Great) would be forced to retreat there in order to launch their counter attacks.
Despite being one of the oldest of Anglo-Saxon Kingdoms, little is known of the Kingdom of East Anglia for it left no written record. It seems to have covered an area close to that of modern Anglia. It’s most famous ruler was Redwald, who is thought to be the king entombed at Sutton Hoo.
The original Anglo-Saxon colony established under Hengest, the Kingdom of Kent was small and mostly unimportant during the 7th-9th century. It never seems to have expanded much beyond the borders of the modern county of Kent. Interestingly, the people of Kent claimed to be Jutes (that is from Jutland) instead of from Saxony like the rest of the Germanic invaders.
Essex (The Kingdom of the East Saxons), Surry, and Sussex (The Kingdom of the South Saxons) also seem to have existed as independent states during the 7th – 9th century though little can be said about them with any certainty.
Bibliography
Ellis, Peter Berresford. Celt and Saxon: The Struggle for Britain AD 410-937. Constable, London. 1993
Morris, John. The Age of Arthur: A History of the British Isles from 350 to 650. Phoenix, 1993
Nicolle, David. Arthur and the Anglo-Saxon Wars. Osprey Publishing. 1984