Carausius was a Menapian who had proven his ability during Maximian's campaign against Bagaudae rebels in Gaul in 286. As a result, he was appointed to command a navy based in the English Channel with the responsiblity to fight raiding Franks and Saxons. Maximian came to suspect Carausius of manipulating events to line his own pockets, and Carausius was forced to flee to Britain around 290, where he declared himself Emperor, and won the alliegance of the three legions based on that island, as well as one in northern Gaul.
As long as Carausius was able to control the southern side of the English Channel, he was safe in Britain. However, as S.S. Frere speculates, Carausius' rebellion endangered Diocletian's vision of a strong, centralized government based on his tetrarchy, and Constantius I was sent to put this rebelion down. By 293, Constantine gained control of northern Gaul and recovered the allegiance of the rebellious Gallic legion. Carausius was shortly afterwards assassinated by Allectus, who succeeded him.