The Evolution of Arthurian Romance
The island was home to a mix of Celtic speaking tribes until the Roman legions invaded in the first century c.e. For 350 years, Romans ruled Britannia as a centralized state. A Roman Balkins-born commander named Lucius Arturius Castis was posted near Hadrian's wall with his Sarmatian soldiers. It was discovered not long ago that these Sarmatians served in England. [Ossetes are (formerly-Soviet-) Turkish survivors of the same tribe, and they have pre-Arthurian folklore of a hero, Batradz, with the same end as Arthur: the women in the barge, etc.] In 185 c.e., the Picts stormed across the wall, and Castis and his cavalry seem to have turned them back.
When the Roman legions were disbanded, fourth- and fifth-century Britannia had minor tyrant kings. Angles, Jutes, and Saxons were frequent invaders, or gradual migrators, and the tyrant Vortigern invited the Saxons to repel the invasions, but these were the most dangerous of the barbarians, so it was a bad time for Britannia.
Gildas -- A monk writing in about the 540s, Gildas the Wise, which would be nearly contemporaneous with the Arthur of history, mentions no Arthur but does refer to a heroic Ambrosius Aurelianus, a military chief and last of the Romans owning great tracts of southern land after the Romans withdrew, who won major victories against Saxon invaders, such as the decisive turning point in the Saxon wars: at Mount Badon around the year 500. Gildas says there was a generation of peace after that.
But the Britons reverted afterwards to separate minor kingdoms. Without a leader to rally them defensively, Saxons and Jutes and the tribes invaded again, forming what they eventually would call Anglo-land. Arthurian legend was important to the colony in Brittany.
Nennius -- Writing in 829, Nennius has Arthur leading the kings of the Britons in twelve battles against germanic invaders, with the last battle being that of Badon Hill. The Britons had the advantage of cavalrymen. Arthur kills 960 warriors. He has a dog named Cabal and carries an image of the Virgin Mary in battle #8. Anir, the son of Arthur, kills Arthur.
"Annales Cambriae" -- This source dates a decisive battle at 537, the battle of Camlann in which Arthur and Medraut fell. This then is another battle noting the death of Arthur.
Arthurian legend came back home from France, because of Britons in exile over the centuries (or Bretons), with the 1066 Norman invasion of Saxon England. At the Battle of Hastings, the left flank was made up of Bretons. Arthur, a fighter against the Anglo-Saxons, was good propaganda, and the sixth-century cavalrymen were recast as knights.
Geoffrey of Monmouth (1100-1154) -- Geoffrey is strongly influenced by Celtic traditions in his prose Latin Historia Regnum Britanniae (The History of the Kings of Britain, c. 1136-1138), blurring history and folktales, even inventing an explanation that his friend Walter, archdeacon of Oxford, loaned him an ancient Welsh source-book. Geoffrey established myth that Britain was founded by Brutus, great-grandson of Aeneas and leader of some other Trojan exiles. Geoffrey also supplies the first references to King Lear and his daughters.
About 1/5 of the book concerns King Arthur. There appear here early forms of Merlin, Utherpendragon (Ambrosius is his brother), Mordred, Lot, Ganhumara (Guinevere), Beduerus (Bedivere), Cajus (Kay), and the Isle of Avalon. Arthur is unwilling to pay tribute to the Roman emperor and marches on Rome, leaving Mordred in charge to turn treasonous, leading to the final battle at Camlann. Arthur dies in 542 and is carried off to the island of Avalon.
Wace (1100-1174) -- In 1155, this Norman cleric translated Geoffrey of Monmouth into French verse: octasyllabic couplets. Wace's Roman de Brut adds the Round Table (early Celtic warriors may have met in circular counsel) and makes Merlin a magician. He is interested in courtly behavior. Arthur's defiance of the Roman emperor Lucius includes first this assertion: "might is not right; it is force and superior power. A man does not possess by right what he has taken by force" -- and then this: "They claim Britain, and I claim Rome. This is the gist of my counsel: that they may have the land and tribute who can take it away from another" (Norton 123).
Layamon's Brut -- Written in the 1190s, this English alliterative verse source, with some rhyme, by the English priest Layamon omits the romance and includes Loth's sons. Prophecies anticipate T.H. White's elaboration of having Merlin change Arthur into animals. The first Grail mention is here, with Arthur descended from Joseph of Arimathea.
12th-century court culture -- chivalry and courtly love -- shaped the legend, and both Camelot and Lancelot came from France. Like Charlemagne in the "Matter of France" romances, Arthur loses his warrior dignity and becomes a master of ceremony. But his successes and his defiance of Rome "served to flatter the self-image and ambitions of the Anglo-Norman barons" (Norton 118).
Other Arthurian material includes The Mabinogion, romances by Chretien, the 14th-century alliterative Morte Darthur, Sir Gawain and the Green Knight, and of course Malory's Le Morte Darthur in the late 15th century; from the Renaissance, The Faerie Queene; the Victorians, Matthew Arnold, William Morris, Swinburne, Tennyson's Idylls of the King; then T.H. White's The Once and Future King; the musical Camelot, Monty Python and the Holy Grail, the wretched film Excalibur.
The Norton Anthology of English Literature, Vol. I. 8th ed. NY: W.W. Norton and Co., 2006.