The historical Ambrosius
A nobleman of Roman origin, who sometime in the period after 441 AD rallied the Romano-British forces to victory against the Anglo-Saxons.
Our source for this is Gildas, who in the De Excidio Britanniae describes the situation after the arrival of the Saxons stating that "the poor remnants of our nation ... being strengthened by God ... took arms under the conduct of Ambrosius Aurelianus" who was able "by the goodness of our Lord" to "obtain the victory"
Gildas further describes him as "a modest man, who of all the Roman nation was then alone in the confusion of this troubled period by chance left alive", and refers to his parents as "who for their merit were adorned with the purple".
The remark that his parents were "adorned with the purple" might be taken to indicate imperial origins, but more likely it suggests the holding of some high office in the Roman administration that predated the revolt of 409 AD.
Although it has also been suggested he came from the same family as Ambrose of Milan who served at the Imperial court earlier in the 4th century.
It is often presumed that this was the same Ambrosius that the Historia Brittonum mentions, when reference is made to Vortigern’s "apprehensions of Ambrosius". This Ambrosius fights and loses against Vortigern’s lieutenant Vitalinius, but subsequently returns after the fall of Vortigern and replaces him as the "great king among the kings of Britain".
Piecing together these fragments one would conjecture that Ambrosius Aurelianus led a rival, pro-Roman party to that of Vortigern’s. After the revolt of the Saxon foederati had discredited Vortigern and his policies, Ambrosius Aurelianus took control, organised armed resistance against the Anglo-Saxons, and achieved some kind of military victory.
What kind of victory, where and when we do not know, as Gildas neglects to give us these details. Clearly it took place before Mons Badonicus, but how long before is unclear, or indeed how long it took Ambrosius Aurelianus to organise the military resources of the Romano-British before this victory was achieved we do not know.
After which Ambrosius Aurelianus drops out of the historical record, such as it is, nothing can be said of the manner or time of his death.
As a candidate for the historical Arthur
There is a line of argument that equates Ambrosius Aurelianus with the historical Arthur, in which case it is generally necessary to assume that the Ambrosius who challenged Vortigern in the period of around 425 to 450 AD was the father of the warrior Ambrosius Aurelianus who was fighting the Anglo-Saxons in the period of 480 to 510 AD.
One can never exclude this possibility, but it seems unlikely. If Ambrosius Aurelianus had led the Romano-British forces in the campaign that culminated in
Mons Badonicus and beyond, one tends to believe that Gildas would have said so, and he does not.
His role in Arthurian legend
The Historia Brittonum contains some obviously mythical elements that portray Ambrosius as a fatherless child with prophetic and almost magical powers.It is Geoffrey of Monmouth develops the story further, calling him Aurelius Ambrosius, and having him fleeing from Vortigern to Armorica, returning to defeat both Vortigern and Hengist King of the Cantware, before meeting his end through poison at the hands of Paschent, one of Vortigern's sons and King of Builth. This Aurelius Ambrosius becomes the brother of Uther Pendragon and therefore the uncle of Arthur.
Part of the Sub-Roman Britain project, where sources are detailed.