Gildas, thought to have been a Scottish cleric educated in
Wales, wrote the polemical De excidio Britanniae (On
the Ruin of Britain) around 540-550, in which he blames the
British for their "ruin" at the hands of the Saxons.
Though he cites the battle at Badon (490? Early 500s?) as decisive--a
battle at which the British apparently achieved a temporary victory
over the encroaching Saxons--he fails to mention Arthur as taking
part in it, let alone leading it.
The first mention of Arthur in the written record is found in
the Gododdin, a series of poetic laments attributed to
the bard Aneirin, and intended to commemorate the battle of Catraeth
(Catterick in Yorkshire?), fought between Britons and Saxon ca.
600. However, while the poem is thought to have been composed
around the time of the battle, it is extant only in a thirteenth-century
manuscript. The lines are:
.[Gwawrddur] fed black ravens on the fortress
[i.e., by killing his enemies]
Though he was no Arthur
The earliest "historical" reference to Arthur is
found in the Historia Brittonum, attributed to Nennius
(thought to have been compiled in the early ninth century; however,
the earliest extant MS is 12th century), in which the "warrior
Arthur" is mentioned as fighting against the Saxons in twelve
great battles.
The Annales Cambriae (The Annals of Wales; compiled
960-80, though the earliest extant MS is much later; one copy
is found in the same miscellany [Harley 3859] as the Historia
Brittonum), states that the battle of Badon took place in
518 (note descrepancy in the date when compared to Gildas), and
that Arthur died at the battle of Camlann in 539. The entries
are:
Year 72 [518?] The battle of Badon in which Arthur bore the Cross
of our Lord Jesus Christ on his shoulders for three days and
nights, and the Britons were the Victors.
Year 93 [539?]
Guieth [Battle of] Camlann in which
Arthur and Medraut [Mordred] perished; and there was plague in
Britain and Ireland.
The next major "historical" source for the story
of Arthur is the Welsh cleric Geoffrey of Monmouth's Historia
Regum Britanniae (History of the Kings of Britain;
completed ca. 1138-39) [assigned, above], along with William
of Malmesbury's Gesta Regum Anglorum (1125) and Henry
of Huntington's Historia Anglorum (1129).
Sources: The New Arthurian Encyclopedia
(gen. ed. Norris Lacy), and Christopher A. Snyder, An Age
of Tyrants: Britain and the Britons, A.D. 400-600.