This theory, proposed by Geoffrey Ashe suggests that the character Arthur existed in history as a Roman-Briton commander, commonly referred to as Riothamus. Riothamus was not the man's actual name however. Riothamus is simply a title meaning "high king". It is synonymous with the title Caesar. Historians argue if Arthur was truly the King of the Britons, or if he was just a commander of British troops. This theory suggests that both scenarios are true. If Arthur did exist as Riothamus, then he was a Roman-Briton commander and at one time was King of the Britons.
Riothamus was a real person that existed in Britain in the late 5th Century. He was referred to as King of the Britons by the current Roman Emperor. Riothamus led an army of Britons from different kingdoms to defend their country against the invading Saxons. Riothamus and his army was victorious in this battle and reigned as high king for several years after. Sidonius Apollinaris sent a letter to Riothamus about a problem with his men that needed to be taken care of. The Roman Emperor at the time also requested Riothamus' help in their fight against the invading Visigoths in Bergundy. Riothamus led 12,000 troops into Gaul and Burgundy to lend help to the Romans. He was betrayed by Arvandus, the prefect of Gaul, and was intercepted by Euric, the King of the Visigoths. Euric and Riothamus fought a gruesome battle but Euric was victorious. After he was defeated, Riothamus fled the battlefield with as many of his troops as he could manage. After this he pretty much disappears in history. The last known whereabouts of Riothamus suggest that he was headed to a Roman stronghold in a town called Avallon. Avallon was a Roman stronghold that was fortified in Burgundy.
The letter from Sidonius to Riothamus is the most persuasive and best proof towards the existence of Artur and his identity as Riothamus. The letter puts Riothamus in a specific place and time period in history. The fact that Riothamus was a title meaning "high king" also lends to most other Arthurian legends about Arthur being the King of the Britons. There are no solid arguments against this theory and it has yet to be disproved. The biggest opposition to the theory was that according to the theory, Riothamus/Arthur existed about 50 years earlier than most other works suggest. Riothamus iis also the only recorded person in history that has done anything Arthurian. He fought against the Saxons to defend Britain. He fought in Gaul. He was betrayed by a confidant that had some relation to him. Finally, he disappeared in Avallon and no there are no records of him being alive or dead.
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