Marcus Aurelius Mausaeus Carausius and during Maximians campaign in Gaul, was chosen to command the Classis Britannica which was a large fleet situated in the English channel.
The purpose of the fleet was to eliminate Saxon and Frankish pirates who had been terrorising the region but Maximian would soon find out that he was allowing the pirates to continue raids, then capture them and keep a large amount of the captured loot for himself. Maximian sentenced him to death and upon hearing about this sentence Carausius declared himself emperor of both Britain and northern Gaul. he had not only his fleet to support him but also three legions stationed in Britain, a legion seized in Gaul along with a group of barbarian mercenaries who were loyal only because of the promise of loot.
In 288, Maximian set about preparing his army to invade Britain but they failed. In 293 though, Carasius was defeated by Constantius Chlorus who was the western Caesar at the time, when he reclaimed Gaul.
With his power dwindling, Allectus who was Carausius’s treasurer saw this as a prime opportunity to take power and murdered him and took the power for himself.