 Constantius met expectations quickly and efficiently and by 293 had expelled Carausian forces from northern Gaul . In the same year , Carausius was assassinated and replaced by his treasurer , Allectus . Constantius marched up the coast to the Rhine and Scheldt estuaries where he was victorious over Carausius ' Frankish allies , taking the title Germanicus maximus . His sights now set on Britain , Constantius spent the following years building an invasion fleet . Maximian , still in Italy after the appointment of Constantius , was apprised of the invasion plans and , in the summer of 296 , returned to Gaul . There , he held the Rhenish frontiers against Carausius ' Frankish allies while Constantius launched his invasion of Britain . Allectus was killed on the North Downs in battle with Constantius ' praetorian prefect , Asclepiodotus . Constantius himself had landed near Dubris ( Dover ) and marched on Londinium ( London ) , whose citizens greeted him as a liberator . 
