Whether this designates a particularly large and prolonged raid, a treaty for establishing federate troops, or a military take-over depends upon one's interpretation of other events. According to Constantius, Britain is a prosperous island. Following the period of prosperity, there is the threat of renewed Pictish raiding, and then a plague. Three keels of Saxons arrive at the invitation of Vortigern, to fight against the Picts and Scots. They are settled in the East of the Island.
Reinforcements subsequently arrive.
It is hard to see how one could infer from this that Riothamus was a king of Brittany, as many authors have. In any case, he confronts the Visigoths in battle but is beaten and retreats into Burgundy. 430 x 525 The Saxon revolt. Britain is plundered, even as far as the west coast. Presumably at least 5, proabably less than 30 years.