Various sources say that they were promised various things. Some sources say that Hengist promised to marry his daughter off to Vortigern. Other sources say that Vortigern promised Hengist all of Kent.

Somewhere along the way, the alliance broke down and the Saxons turned to fighting the Britons instead.

The Anglo-Saxon Chronicle says that this breakdown in relations happened in 455, just six years after the Saxon landing. A Chronicle entry for two years after that says Hengist and his forces killed 4,000 Britons, driving them out of Kent entirely.

Chronicle entries that follow talk of further Anglo-Saxon victories against Britons and Welsh. The entry for 465 says that the Anglo-Saxons killed 12 Welsh leaders, presumably as well as the men who served with them. The entry for 473 records further fighting, after which "the Welsh fled … like fire."

The picture that emerges from these descriptions is of a force that starts with a beachhead on the southeastern part of the British Isles and then drives its way westward, staying in the southern part of the country and driving its enemy all the way to the Welsh border and through it, forcing the Welsh who had come to the Britons' aid to abandon the fighting altogether in the face of a superior enemy. This is reminiscent of the Roman conquest of a few centuries before.

But where is Vortigern in all this? Depending on the source, he was either doing his level best and was just overwhelmed by events or he was incompetent and deserved the fate he got.

What he got was a rather ignoble death, after which subsequent chroniclers placed much of the blame on him for the Adventus Saxonum, as Gildas termed the coming of the Saxon peoples.

Vortigern has at times been depicted as weak and indecisive, scheming and manipulative, wealthy and influential, and immoral and opportunistic. He is not described as a warrior. One of his sons, Vortimer, was said to be in charge of the army, and more than one source says that Vortimer led the revolt against his own father when the Britons thought that things had gotten out of hand.

Was Vortigern a civil governor? He was certainly a well-known figure of the time. If he was an "overlord," as one interpretation has it, he must have had some way of rallying people to his cause. Even if he was the "first among equals," as one theorist has it, he was still the spokesman appointed by the council to extend overtures to the Saxons.

He wasn't powerful enough to keep things under control, though. Once Hengist and Horsa turned on Vortigern, there was no stopping them.