Actually, no. They left when the Roman governor of Britain decided that he should be Emperor, and he took the Legions with him. After his rebellion was defeated, the Legions were never replaced.
That left the coasts clear (literally) for the Saxon pirates to raid at will. One of the local British kings invited a Saxon chief to take over the defense of his kingdom in return for land. The Saxons then just moved in and took over.
According to a lot of the old Welsh history, I think that was Vortigern who offered Hengist the security contract for his kingdom, with land as a major perq. Over less than a century, Hengist and his descendants brought over pretty much half of the Saxon population, and secured Vortigern’s place in British history as everything but the antichrist. I’ve read about it in British history books since my teens, and I still can’t understand his motivation, other than the fact that he wasn’t well liked for whatever reason, so no one in Britain would help him.
Interesting. I’ll be brushing up my history a bit.
Thanks.