In which the fifth century Romano-British ruler Vortigern hired some Saxon federates or foederati to defend the kingdom against Pictish raids from the North, possible Roman re-occupation and to bolster his domestic powerbase. Sometime afterwards at least some or perhaps all of these foederati rebelled. The subsequent conflict is often called The War of the Saxon Federates.
The dating of the rebellion
The exact date for the rebellion is uncertain (as is much else from this period), but we know from the evidence of the de Vita Germani of the visit of Saint Germanus to Britain in 429 AD. Which is important as it tells us that there was no war in 429, and that whenever the rebellion occurred, it occurred after this date.
The location of the rebellion
Tradition, specifically the tales of Hengist and Horsa place the rebellion in the tribal territories of the Cantii, or what become known as Kent. Indeed the archeological evidence from a number of sites suggests the deliberate posting of foederati to protect Londinium.
But we should also remember that logic dictates that if these federates were hired to protect the kingdom from Pictish and Scottish incursions, some of them at least must have been based in the north of Britain. The Historia Britonnum tell us that Hengist supposedly sends for his two sons Octa and Ebusa, who arrive with forty ships.
In these they sailed round the country of the Picts, laid waste the Orkneys, and took possession of many regions, even to the Pictish confines.
Perhaps; but certainly there were
Anglo-Saxon foederati somewhere in the north, although whether these northern mercenaries participated in the intital rebellion is simply not known.
The evidence for the rebellion itself
As described by Gildas in the De Excidio Britanniae
The barbarians being thus introduced as soldiers into the island...they complain that their monthly supplies are not furnished in sufficient abundance, and they industriously aggravate each occasion of quarrel, saying that unless more liberality is shown them, they will break the treaty and plunder the whole island. In a short time, they follow up their threats with deeds.
But true to form, Gildas is not interested in providing us with any actual detail as to when and where this occurred.
Nennius's accounts of Vortimer
The Historia Britonnum recounts us with the tales of Vortimer, son of Vortigern, and his battles against Hengist and Horsa, successfully pushing the invaders back into Kent. But after Vortimer's death "the barbarians became firmly incorporated" and send back to Germany for reinforcements.
The Anglo-Saxon Chroncile records a few battles in the years between 455 and 477 with the customary exaggerated body count, but is otherwise of little assistance. It confirms the tradition of the Kentish rebellion but little else.
How much of the above can we believe?
We can believe that {Vortigern] existed, that he hired his foederati, that they rebelled, and that the war took place. We can except that the focus of the war was in the extreme south east of Britain and we can infer from subsequent events that the Romano-British failed to quash the rebellion; a failure which led to a much wider struggle for mastery of the island of Britain
Part of the Sub-Roman Britain project, where sources are detailed.