The Sarmatian Hypothesis proposes that the original, historical Arthur was a Roman military leader named Lucius Artorius Castus, who commanded of a group of Sarmatian warriors for two years in Britain, around 182. Castus probably led this group in battle several times, though he left Britain for other commands and eventually died in Dalmatia. The theory proposes that the Sarmatians in Britain continued to live there, possibly employed by the Roman military, possibly maintaining their nomadic ways and their own language, but at any rate telling their traditional stories with a new hero, Castus, called by them Artorius.
Although the stories of Arthur were incorporated into British history as those of a fifth/sixth-century British military leader such as in Historia Brittonum or king as described by Geoffrey of Monmouth leading a confederation of British kings against the Germanic invaders/colonists, they are thought to contain information about the actual brief career of Castus in Britain, or perhaps of subsequent leaders of the Sarmatians nicknamed or actually named Artorius. According to this hypothesis, the emphasis on horsemanship in the later legends does not reflect high medieval chivalry but rather the horsemanship of these steppe nomads.
However, it is important to note that most of the parallels and similarities between Arthurian and Sarmatian tales are found in writings dating from and after Geoffrey of Monmouth and thus do not apply to the core issues of historicity. Thomas Malory’s Le Morte d’Arthur bears the strongest similarities to the Sarmatian tales, but by this time, the Arthurian knights had evolved into “knights in shining armour”. Critics have thus concluded that the Sarmatian stories relate to the post-Galfridian development of Arthurian tales and not to the historical basis.