A people or tribe of uncertain origin that established a kingdom in the Thames Valley area of Britain during the sixth and seventh centuries AD.
As the Venerable Bede wrote,
gens Occidentalium Saxonum gui antiquitus Geuissae uocabantur
or in English,
the people of the West Saxons who in the past were called Gewissae.
There is no agreed derivation of the name, traditionally the answer has been that it is a reference to descent from one "Gewis", based on the genealogy quoted in the Anglo-Saxon Chronicle,
Cerdic was the son of Elesa, Elesa of Esla, Esla of Gewis, Gewis of Wye, Wye of Frewin, Frewin of Frithgar, Frithgar of Brand, Brand of Balday, Balday of Woden.
The kings of the Gewissae
A traditional king list for the Gewissae goes something like this, (1)
The odd thing about many of these names is that they don't seem very Anglo-Saxon, they are in fact slightly Germanised versions of Brythonic, Celtic names; Cerdic is Ceredig or Caractacus, Cyneglis is Cynglas or Cuneglasus, and perhaps most surprising of all is finding a supposedly Saxon king called Caedwalla. Surprising because his namesake Cadwallon ap Cadfan was not long before busy "wipe out the whole of the English nation from the land of Britain" according to Bede.
Which strongly suggests some sort of joint Brythonic and Germanic origin for the Gewissae, (3), and idea that will always retain a degree of controversy as a great number of English historians are naturally uncomfortable with such an idea.
Where did the Gewissae come from?
The archeological evidence shows the presence of Germanic troops very early in the fifth century in the area of the upper Thames, implying that they were there even before the Romano-British leader Vortigern hired his Jutish mercenaries around 429; and certainly there before any conventional dating for the arrival of the Saxons.
Possibly implying a Germanic cohort of the Roman Army that remained behind and had been entrusted with the defence of the Thames Valley area. The name Gewissae might even be derived from the same root as Gwynedd or Gwent and indicate troops that had previously been stationed at Segontium or Venta.
The truth is that we shall never really know for certain as there is no historical record to guide us.
What do we know about these Gewissae?
It is certain that these Gewissae established a kingdom based in the Thames Valley with their capital at Dorchester-on-Thames early in the sixth century. They expanded westwards in the latter half of the sixth century; in 552 they fought the Britons at Salisbury, in 556 it was Banbury and at the Battle of Dyrham in around 577 they gained the towns of Bath, Gloucester and Cirencester, afterwards known as the territory of the Hwicce.
The Anglo-Saxon Chronicle also has them fighting the kings of Kent as well as the South Saxons and says of Ceolwulf that "he constantly fought and conquered, either with the Angles, or the Welsh, or the Picts, or the Scots."
But the story became one of increasing pressure from Mercia; in 591 Ceawlin was driven from his kingdom, and in 628 Cynegils fought Penda at Cirencester; the Anglo-Saxon Chronicle says that they entered into a treaty, by which the territory of the Hwicce seems to have been surrendered to Mercia.
The Gewissae seem to sought an alliance with Oswald of Northumbria, an alliance that was made easier by their adoption of Christianity as a result of the arrival of the bishop Birinius (4) who converted the king Cynegils and established his episcopal see at Dorchester-on-Thames in 636.
However Oswald's subsequent invasion of Mercia came unstuck at the battle of Maserfeld in 642, and the alliance seems to have brought little relief to the Gewissae. In 645 Cenwalh was driven from the kingdom by Penda of Mercia and spent three years in exile with the East Saxons (5), and again in 661 Cenwalh fought (and lost) with the Mercians at Pontesbury. By 685 his successor Ceadwalla "began to struggle for a kingdom". (As the Anglo-Saxon Chronicle puts it.)
At which point Ceadwalla led the Gewissae south where they reinvented themselves as the West Saxons and established the kingdom of Wessex.
(1) The gaps in the chronology arising as a result of Merican incursion and domination.
(2) The Anglo-Saxon Chronicle says that after the death of Cenwalh that his queen Sexburga "held the government one year after him ", but this tends to get omitted from the traditional lists for whatever reason.
(3) A theory first advanced by JNL Myers who suggested that Cerdic was the head of a Romano-British noble family, with blood ties to existing Germanic settlers, and supported by others, for example see R. Coates, On Some Controversy surrounding Gewissae/Gewissei, Cerdic and Ceawlin Nomina, xiii (1989-90)
(4)According to Bede, Birinius was on a mission to,
sow the seed of the holy faith in the inner parts beyond the dominions of the English...but on his arrival in Britain, he first entered the nation of the Gewissae, and finding all there most confirmed pagans, he thought it better to preach the word of God there, than to proceed further to seek for others to preach to.
Which seems rather odd on the face of it, as the "inner parts beyond the dominions " would have been, well, Welsh and they already had their own Celtic Church and were not "confirmed pagans". Perhaps it indicates that Birinius' real mission was to try and bring the Celtic Church back within the orbit of control of the Rome and the see of Canterbury and that his real achievement was to do exactly that with whatever remnants of Romano-British Christianity he found within the kingdom of the Gewissae.
(5)It is said that this was on account of having discarded Penda's sister whom Cenwalh had previously married and taken another wife.