Whatever the exact outcome of the War of the Saxon Federates, this initial rebellion of the Anglo-Saxon foederati or mercenaries led to a much wider struggle for the control of Britain itself.
The exact details of exactly what happened are lost in the mists of time, but is more than likely that once the rebellion had demonstrated the vulnerability of the Romano-British that word would have spread of the rich pickings to be had in the former Britannia. As the Anglo-Saxon Chronicle suggests, more Anglo-Saxons arrived, eager for conquest and booty.
Let us see if we can construct some sort of vaguely consistent chronicle of what might have happened.
The evidence of the Gallic Chronicles
We can refer to the entry for the year 441 from the Gallic Chronicle of 452,
Britain, which had up to this time suffered various defeats and catastrophes, was reduced to Saxon rule
The evidence of De Excidio Britanniae
Gildas echoes the bald statement of the the Gallic Chronicles with a more poetic description of events,
Lamentable to behold, in the midst of the streets lay the tops of lofty towers, tumbled to the ground, stones of high walls, holy altars, fragments of human bodies, covered with livid clots of coagulated blood, looking as if they had been squeezed together in a press; and with no chance of being buried, save in the ruins of the houses, or in the ravening bellies of wild beasts and birds..
He then tells us that it was Ambrosius Aurelianus who initially rallied the Romano-British forces and achieved the victory and that,
After this, sometimes our countrymen, sometimes the enemy, won the field, until the year of the siege of Mons Badonicus, when took place also the last almost, though not the least slaughter of our cruel foes
Unfortunately that is all the detail that Gildas
gives us concerning the conduct of the war. We might wish for more, but Gildas
is more interested in getting down to the meat of his sermon rather than telling us what actually happened.
Nennius's accounts of Arthur
The Historia Brittonum fills in the blanks with the following,
Then it was, that the magnanimous Arthur, with all the kings and military force of Britain, fought against the Saxons. And though there were many more noble than himself, yet he was twelve times chosen their commander, and was as often conqueror.
and lists the total of twelve battles
fought against the invaders.
The end of the war
The archeological evidence shows a halt to Anglo-Saxon settlements towards the end of the fifth century (To the extent that there is evidence of Anglo-Saxon emmigration from Britain back to the continent.)
In the end neither side were ultimately victorious; the Romano-British failed to expel the Anglo-Saxon invaders, and the Anglo-Saxons in turn failed to conquer Britannia. A stalemate was reached with embryonic Anglo-Saxon kingdoms established along the southern and eastern coasts of Britain, whilst the north and west remained
So what actually happened?
We can believe that there certainly was a struggle for the control of the island of Britain. When it began is uncertain, when it ended is uncertain but if, in all probability, the Anglo-Saxons were rampaging all over Britannia by 441 AD, and Mons Badonicus was fought in 496 AD (or thereabouts), not to mention the fighting afterwards, then we are talking about a long war.
Secondly, we can accept that the effect was pretty devestating for the defenders, as the structure of Roman Britain was effectively demolished by the war. As Gildas puts it,
And yet neither to this day are the cities of our country inhabited as before, but being forsaken and overthrown, still lie desolate; our foreign wars having ceased, but our civil troubles still remaining.
But as do the detail of the conflict nothing can be said. Even if one were to place some credence on Nennius's twelve battles; their location is disputed, their date unknown. All we really have is the battle of Mons Badonicus. Even the exact details regarding the cessation of the war are unknown. We cannot be entirely certain where the boundaries between the respective Anglo-Saxon and Romano-British kingdoms lay; we rely on what can be inferred from the seventh century historical record and the tentative insights of archeology.
We should note the words of Gildas who refers not only to the "terrible desolation of the island" but also to the "unexpected recovery of the same". Indeed there was something faintly miraculous(1) about the whole thing; across the whole of the Western Roman Empire provinces tumbled before the barbarian incursions, Rome itself fell in 476. But here in the remotest corner of the former empire, the inhabitants somehow summoned the will to resist, and successfully fought back against the invaders.
Part of the Sub-Roman Britain project, where sources are detailed.
(1) It is not surprising in many ways therefore that people have sought to explain this minor miracle by means of the heroic figure of King Arthur. Howver to those who wish to believe that there was an historical Arthur there remains one central problem, he is not named by Gildas.