Aelle was king of the South Saxons from 477 to perhaps as late as 514, and was named Bretwalda by Bede, who adds that he was overlord of the Anglo-Saxons south of the Humber river.
Our primary source for the events of Ælle's life (besides the short mention in Bede's Ecclesiastical History) is the Anglo-Saxon Chronicle. It states that he landed in Britain in 477 with three ships and his three sons Cymen, Wlencing, and Cissa at Cymenes ora, where "they killed many of the Welsh, and drove the rest into the wood that is called Anredsleage." For the year 485, the Chronicle records that he again fought the "Welsh" at the stream of Mearcread. Then in 491, Ælle with the help of Cissa successfully besieged Anderida (also identified as Pevensey), and slew all of the inhabitants. E. A. Thompson notes that this is the only example of the barbarians who invaded the Roman Empire successfully using siegecraft against a Roman city! And with that last entry, the Chronicle contains no more records of this warchief; we have no record when he died, nor how, nor even what happened in the kingdom of the South Saxons after his presumed death until the baptism of its king Æthelwalh around 675.
The Anglo-Saxon Chronicle at this point begins narrating the events of the founding of the West Saxon kingdom, or Wessex, so it is possible that the scribe assembling this chronicle forgot to return to the events of Ælle's life. Alaistar Campbell, in examining the chronology of this part of the Chronicle, notes that at several places events are duplicated at 28 year intervals, suggesting that the sources from which the composing scribe assembled the Chronicle were based on 28-year Easter Tables, and that the annal that mentioned the later events of Ælle's life were mislaid.
It has been suggested that Ælle led the Anglo-Saxon army at the Battle of Mons Badonicus, possibly as early as 496, though the Annales Cambriae in the Historia Brittonum records the date as 516), and some scholars wonder if Ælle was killed in the battle. This would be a fitting end to the career of the first Bretwalda.