http://en.wikipedia.org/wiki/Great_Conspiracy
The garrison of Hadrian’s Wall rebelled in 367 allying with Saxons and Goths.
Thanks S.H. By that time, they probably *were* Saxons and Goths. The Roman regular army (which had begun to take on more of the character of non-Roman auxiliary legions pretty early on) was withdrawn from Britain for good about forty years after this date; in the 3rd c the Roman Empire had been politically divided into a number of different independent states, including an independent Britain. This was mostly a response to the lack of orderly succession and a continuous might makes right approach to things. It’s strange to think that the Roman Empire “fell” a lot of times before it was gone even in name. Also shows that external threats were not very serious or at least not considered serious by polities — no doubt because the ethnic character of the provinces had itself changed during Roman rule.