The Battle of Badon Hill
ca. 500 AD
Either a siege or a battle (depending on who you read), which effectively defeated the Saxons until the end of Arthur's reign. (If there is an Arthur, which there probably was.) According to Gildas, it took place in the year of his birth, and was lead by Ambrosius Aurelianus.
"...that they might not be brought to utter destruction, took arms under the conduct of Ambrosius Aurelianus, a modest man, who of all the Roman nation was then alone in the confusion of this troubled period by chance left alive. His parents, who for their merit were adorned with the purple, had been slain in these same broils, and now his progeny in these our days, although shamefully degenerated from the worthiness of their ancestors, provoke to battle their cruel conquerors, and by the goodness of our Lord obtain the victory. After this, sometimes our countrymen, sometimes the enemy, won the field, to the end that our Lord might in this land try after his accustomed manner these his Israelites, whether they loved him or not, until the year of the siege of Mount Badon, when took place also the last almost, though not the least slaughter of our cruel foes, which was (as I am sure) forty-four years and one month after the landing of the Saxons, and also the time of my own nativity." --St. Gildas, Ch 24-25 of De Excidio Britainnae
Gildas makes no mention of Arthur, though he does mention a man named "The Bear" (which essentially is what "Arthur" means--"Bear Man"). He also doesn't mention who won the battle.
It was Nennius who says that Arthur was the champion of the battle:
"The twelfth battle was on Badon Hill and in it nine hundred and sixty men fell in one day, from a single charge of Arthur's, and no-one lay them low save he alone." --Nennius
The peace which followed is thought to have lasted fifty years, until the plague of 547 or so, which killed much of the island, leaving it vulnerable to attack.
But where is Badon Hill? Geoffrey says it's Bath, which is possible, as "Baddon" in Welsh would be pronounced "BATH-un" (the TH being like TH in "leather"). Others say Wychbury Hill in the West Midlands, a Badon Hill in Linlithgow, Scotland (if one takes the "Northern Arthur" approach), and so on. Personally, I somewhat hold to the Bath theory, or somewhere in the vacinity, as I hold to a Southern Arthur view, what with his likely Roman decent. But this is all open to debate.