The kingdom of Wessex was one of the most important kingdoms in early medieval Britain. Its ruling dynasty eventually became the rulers of all England. The kingdom was originally settled by Saxons; the name "Wessex" derives from "West Saxon."
The original center of Wessex, which persisted throughout the early Middle Ages, was made up of the present day counties of Hampshire, Dorset, Somerset and Wiltshire. The boundaries of the kingdom fluctuated, sometimes reaching north of the Thames River, and eventually expanding westward to include Cornwall and Devon.
Two settlements formed the core from which the kingdom of Wessex sprang. The first, according to the Anglo-Saxon Chronicle, was founded by Cerdic and his son (or, possibly, his grandson) Cynric, when they landed in what is now Hampshire in 494 or 495. The second is known only from archaeological evidence and was located on the upper Thames. This latter settlement was supposedly captured from the British by Cuthwulf in 571, but archaeological evidence indicates an earlier Saxon presence.
Much of the early history of Wessex and its expansion as a kingdom is obscure. But some events were recorded in the Anglo-Saxon Chronicle, which relates several conquests of Cerdic and Cynric, wherein they drove away the Celts, or Britons, who had occupied those lands. The Chronicle places the date of their becoming kings (when they "undertook the government of the West-Saxons") at 519. When Cerdic died in 534, Cynric ruled alone for more than two decades.
In 560, Cynric was succeeded by his son Ceawlin, who consolidated several Saxon communities in the Chiltern Hills into the kingdom. He also defeated the Celts in some significant battles, expanding the territory of Wessex north of the Thames. Ceawlin is mentioned by the Venerable Bede in his Ecclesiastical History of England as the second English king to hold an imperium in Britain. In 592, Ceawlin was expelled by his nephew Ceol.
In contrast to his predecessors' long reigns, Ceol only ruled for five years before his brother Ceolwulf succeeded him; Ceolwulf was in turn succeeded by Ceol's son Cynegils. According to the Anglo-Saxon Chronicle, when Cynegils was baptized in 635, King Oswald of Northumbria was his sponsor; this seems to indicate a lessening of Wessex' aggressive expansion. And, indeed, Cynegils lost Gloucestershire, Worcestershire, and part of Warwickshire to King Penda of Mercia. Cenwalh, son and successor to Cynegils, married Penda's sister; but not long after he abandoned her, for which Penda drove him into exile in East Anglia for three years.
Cenwalh did not sit idle for long; he spent much of his reign fighting the Welsh as well as the Mercians. Unfortunately, he lost the Isle of Wight and South Hampshire to King Paeda, Penda's son and successor. According to Bede, after Cenwalh's death much of Wessex was divided into sub-kingdoms. Parts were under the control of Mercia until 686.
Still, it wasn't long before Wessex kings began to expand the kingdonm once more. Ceadwalla, who reigned from 685 to 688, recovered South Hampshire and the Isle of Wight. His successor, Ine (or Ina, or Ini), held strong against the Cornish Britons and the South Saxons until he retired to Rome in 726, where he died shortly thereafter. Ine was the first king of Wessex to issue a law code. The kings who followed him managed to keep the kingdom more or less intact, though much of it was subject to Mercian control.
In 802 King Egbert acceded to the throne. Egbert put an end to Mercian dominance, establishing himself as an independent ruler of Wessex. He recovered Devon and Cornwall and defeated Beornwulf of Mercia in 825, bringing Kent, Surrey and Sussex under Wessex rule. In 829 he would be proclaimed "ruler of all Britain"; and though he lost Mercia to Wiglaf in 830, the strengthening of Egbert's kingdom would make Wessex an excellent core for future England.
The final triumph of Wessex would come with the victories of Alfred the Great, who fended off invading Danes and consolidated the remains of the other kingdoms in the Heptarchy under one rule. From thenceforward, the story of Wessex was the story of all England.