The Battle of Mount Badon was a major victory of the British over the Saxons, and has been part of the Arthurian narrative since the very beginning. The historicity of the actual battle is open for debate, but Badon nevertheless has an important place in the Arthurian literary tradition even prior to Geoffrey of Monmouth's popularization of Arthur.
Gildas and Bede both wrote about the battle but it was Nennius who first wrote of King Arthur being involved. Nennius makes Badon the culmination of a series of twelve battles, even claiming that Arthur personally killed nine hundred and sixty Saxons in a single charge.
According to Geoffrey of Monmouth, Arthur and Cheldric agreed that the Saxons would leave Britain. The Saxons renege on the deal, changing course halfway to France and returning to wreak havoc on English shores. Arthur expresses his outrage at the insult to his honor, and abandons his campaign in Scotland to go down and vanquish the Saxon hordes.
The early stories' account that the Saxons were thrown back around this time seems to be supported by archaeological evidence. Studies of cemeteries suggests the border shifted some time around 500 AD, a date agreed with by Gildas.
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A comprehensive essay on the battle, by Sam Boyer can be read here.