450 Hengest and Horsa in Kent
British king Vortigern granted Saxon raiders Hengest and his brother Horsa the Isle of Thanet in Kent. In exchange, the two brothers were to protect England from the Picts. Hengest and Horsa repelled the Picts and returned to the Isle of Thanet. Now firm allies, Vortigern married Hengest’s daughter and in exchange was granted Kent for the Saxons. Vortigern had not, unfortunately, consulted with the ruler of Kent. Realizing this might be a potentially sticky situation, Vortigern fled north with his wives.
Vortigern’s son Vortimer stayed and fought the Saxons. He enjoyed some initial success and drove the Saxons back to the Isle of Thanet, but was eventually defeated by Hengest and Horsa at Aylesford, Kent in 455. Horsa was killed in the battle and and Hengest offered peace to Vortigern. However, this was just a ruse and when the British leaders were assembled, Hengest had them all murdered except Vortigern, who was held for ransom. Some Britons fought on, but in 457 Hengest and his son Æsc comprehensively defeated the Britons at Crayford and Kent was abandoned to the Saxons. In exchange for his life, Vortigern granted more land to the Saxons, who established kingdoms for the East Saxons, Middle Saxons and South Saxons. These kingdons became the basis for the modern counties of Essex, Middlesex and Sussex.
473 Ælle, First King of Sussex
Consolidating their new land grants from Vortigern, the Saxons started raiding east, down the coast from Kent. In 473 Ælle and sons Cymen, Wlencing and Cissa landed at Selsey which they named Cymen's shore. They fought the Britons and drove them into the weald.
Ælle landed again in 485 and fought against the Britons near the margin of Mearcred's Burn. This site has not been identified as yet, but it is most likely in Sussex. There is also some speculation that this may have been an earlier unsuccessful attempt by Ælle to take Pevensey.
491 The Siege of Pevensey
In 491 Ælle and Cissa landed at Anderitum (Pevensey) and lay siege to the Roman fort. Initially Ælle was harried by Britons attacking from the weald. Ælle split his force. Leaving enough men to hold the siege, he used his main force to drive off the Britons back into the weald. Anderitum eventually fell and Ælle killed all the survivors.
495 The Battle of Badon Hill
Having conquered Sussex, Ælle left his youngest son Cissa to govern Sussex and pressed further west. The British had rallied under general Ambrosius, an anglo-roman who had remained in Britain after the Romans had abandoned the island. (Ambrosius is thought by some to be the basis for the King Arthur legends.) Ambrosius reportedly fought and defeated the Saxons twelve times, with the final victory at Badon Hill.
Badon Hill was to be the last major British victory over the Saxons. It was fought at an unknown site in the southwest of England, but possibly near Bath. Historian Bede refers to Ælle as the first saxon over-king and it is possible that Ælle was the defeated Saxon leader at that battle. There is little subsequent historical reference to Ælle or any other major Saxon leaders from Sussex. This would be consistent with Ælle and his chief lieutenants meeting their end in that battle.
Aftermath of the Battle
There is no further record of the British trying to drive the Saxons back from the British shores. While Badon Hill was recorded a British victory, it may be that there were high losses on both sides and the British did not have the forces or the appetite to wholly drive the Saxons from Britain. It is quite possible that a truce between the British and the Saxons who were already settled was agreed, allowing Cissa to continue is reign as King of Sussex.
There continued a breif period of peace until the Saxons once again started raiding in 508.