Vortigern’s deal with Hengist – you give me soldiers to fight my enemies and I’ll give you Thanet to settle in – was not the crazy and disastrous choice Gildas has us believe. In NW Europe in the 440s, such deals were normal.
As warlord of SE Britannia, Vortigern had no better choice. Aetius, Rome’s powerful man in western Europe, had done just such a deal with the Alans in 440-442 and another with the Burgundians in 443. A few years later he made a similar agreement with Attila the Hun.
In 446, Rome under Aetius was fighting on three fronts – NW Iberia, Brittany, and Belgium. He hadn’t got men to spare for Britain. Rome didn’t have an army other than its foederati, hired warriors like the Alans – and Hengist. Aetius could even have advocated Hengist to Vortigern as an ally. Rome-lover Gildas would not have published that even if he knew it to be true.
Attila the Hun changed sides in 451 and attacked Gaul. Hengist followed Attila’s example, expanding his power in Britain by fair means and foul at Vortigern’s expense.
However, the Historia Britonnum records that Vortigern’s son Vortimer regained all of Kent. The Anglo-Saxon Chronicle of course does not mention this, but its dates of what is in there suggest that Vortimer’s successes were in the 460s, his final driving of the Saxons out of Kent most likely happening in c465-468. So the results of Vortigern’s deal in 446 were not perdurable; it was events after 468 that came to gradually determine the future of what, centuries later, would be England.
In symbolic language typical of the culture of fifth century Britons, it is reported that when he died, five years after his last victory, Vortimer instructed that his head be displayed facing the ocean at Richborough; and that this was not done. Decoded, the meaning is, at minimum, that the Saxons came back: Vortimer’s successes, alas, were also ephemeral. His severed head should have been a talisman threatening any Saxon who dared to attack Kent, but it (symbolically), or more literally the Kentish defences militarily, was inadequate.
Only in outline is the next stage of British history clear. The exact political structure is not known, but it is almost safe to say that eastern coastal districts remained under Anglo-Saxon control from Deira (East Yorkshire) south as far as East Anglia, and probably Essex too. In 473, Saxons were able to slaughter citizens and carry off booty in the largest Saxon raid of the entire fifth century. After that calamity, Ambrosius Aurelianus, presumably Governor of Britannia Prima, perhaps a descendant of the Roman Emperor Aurelianus (270-275), became chief warlord of southern Britain, and led the Britons valiantly against the Saxons, with some measure both of unity and of success.
The presence of a line of Ambrs- place-names running north-east from Amersham suggests that he held a line of defence there against English attackers coming from Essex and East Anglia. The northern Home Counties were successfully defended and became the Kingdom of Calchfynydd (meaning: Chalk Hills i.e. Chliterns).
Two other Ambrs on the South Downs in west Sussex (Ambersham near Midhurst and Amberley to the north of Arundel) and one in Kent (guarding the Medway crossing somewhere near Maidstone) suggest that he also fought effectively in that area. In this context it is significant that the Anglo-Saxon Chronicle identifies battles in Sussex in 477, 485, and 490. It implies that the Saxon side won, though not decisively in 477; silently hints at the probability that they lost in 485; and claims a decisive victory in 490.
490 (or possibly early 491) of course was also the year of the famous British victory at Mount Badon, attributed to King Arthur. Gildas implies that there was when he wrote his “Ruin and Conquest of Britain” (c530) an agreed frontier between the Britons and “our enemies in the east”. It is credible to suppose that the fall of the fort of Pevensey resulted in this frontier being negotiated in c491 as a cease-fire line with Sussex on the Saxon side and Calchfynydd on the Britons’ side.
One can only speculate about the possibility of a Briton-ruled salient between the North Downs and the Thames. The place-name Eccles is elsewhere associated with sixth-century Christian presence under Briton control; there is an Eccles north of Maidstone. Other Christian-suggestive place-names further west in this putative salient include Esher, Godstone, and Godalming.
In any case, the frontier kept its stability for 60 years. But it began to frazzle about 550; and the definitive English conquest of Britain came in the 570s when four calamitous battles (571, Bedford; 577, Dyrham, above Bath; 578, York; and 580, Weedon Bec, Northants) pulled many rich lands of what was once Britannia into the Anglo-Saxon kingdoms. Before 570, Britain was an essentially Celtic-ruled island, despite the presence of English-ruled kingdoms around the eastern and southern coasts. After 580, whatever remained under Celtic control would never again be more than a “Celtic fringe”. The fruits of battle victories in the fifth century were ever precarious and temporary; it was those Angle and Saxon victories in the 570s that were decisive, setting Britain on the never-reversed course for its English future.