Constantine moved quickly. He crossed the Channel at Bononia (Boulogne) and (historians have assumed) took with him all of the mobile troops left in Britain, thus denuding the province of any first line military protection and explaining the disappearance of the legions from Britannia in the early fifth century. The Roman forces in Gaul (modern France) declared for him, followed by most of those in Hispania (modern Spain). On 31 December 406 several tribes of barbarian invaders, including the Vandals, the Burgundians, the Alans and the Sueves had crossed the Rhine, perhaps near Mainz, and overrun the Roman defensive works in a successful invasion of the Western Roman Empire.