@oralloy,
Ambrosius Aurelianus is the most likely candidate. Although I don't know the credentialsoof the author or authors of the linked Wikipedia source, three are several things with which I do not agree. Ambrosius is mentioned in at leasst two of the four (or five) Gallic chronicles which are contemporary or nearly contemporary to the battle of Badon Hill (Mons Badonicus) mention Ambrosius, but not the battle. Gildas, then nearest actual Briton in time to mention the battle (he stated tat he was born in Britain in the same year as the battle, which gives a close approximation of the date of the battle), does not mention who commanded there. He praises Ambrosius, but his work,
De Excidio et Conquestu Britanniae (On the Ruin--or Corruption--of Britain) was, effectively a polemic, a sermon to the effect that the moral failings of the British "Princes" (i.e., Chieftains) was the cause of their ruin, as God punished them by allowing the pagan Saxons to defeat them. Bede, who wrote
The Ecclesiastical History of the English People seems clearly to have followed Gildas, the only near contemporary source. Gildas never mentions any "King Arthur." Nennius wrote
Historia Brittonum (History of the Britons), in the early 9th century according to an early 10th century prologue attached to the work.. Another document is attributed to Nennius, and "appendix" of the twelve great victories of "King Arthur." But such a document is not mentioned in the 10th century prologue which identifies Nennius. No copy of this alleged appendix has been found any earlier than the 14th century--at which point one is more than eight centuries beyond when the putative Arthur is supposed to have flourished, and five centuries beyond when Nennius wrote his history. I consider it to be very suspect. In the History of the British, Nennius does not mention any King Arthur, and the great victory at Badon Hill would have been only about three centuries earlier. "King Arthur" is not mentioned in any contemporary or near-contemporary source on the continent, although Ambrosius is, and the visit of Bishop Germans of Auxerre to Britain is prominently mentioned by Gildas and also mentioned in Gallic sources.
No Welsh source which mentions "King Arthur" exists any earlier than the 14th century. Chrétien de Troyes is the first person known positively to have written about "King Arthur" and he probably created the character Lancelot--that name is certainly not British. Finally, Arthur is not a British name, either. People who have studied enough to know that, but still want to believe in a "King Arthur" come up with all manner of tortured Brythonic sources for the name. The Irish raided into the island, both north and south of what we now call Wales. Artuir
is an Irish name, and more than one of them appears in legends and chronicles by the Welsh.
From the time of Chrétien de Troyes (late 12th century), stories about "King Arthur" became increasingly popular. Marie de France, who was a contemporary of de Troyes, who wrote such stories for the "English" court of Henry II Plantagenet and his Queen consort, Eleanor of Aquitaine, never mentions any "King Arthur," nor any of the knights alleged to have coalesced around him. That is a very loud silence. But by the 13th and 14th centuries, the stories were extremely popular in France, and increasingly so in England. The monk in charge of a monastic scriptorium could make good money churning out copies of such stories, and the more spectacular, the better. Geoffrey of Monmouth wrote a history of the kings of Britain in the 12th century which is just silly with its extravagant claims. All the more reason thet the silence of Marie de France is so "loud." I strongly suspect he was writing for a continental audience. Monmouth as his Arthur conquering Iceland, Scotland, Ireland, and overrunning Gaul, and finally defeating the Roman imperial armies in the west.
All of which leads me to call bullsh*t on the entire Arthur story. Entertaining? Yes, if you like that sort of thing. Believable? Not a bit.