In 286/7 Carausius commander of the Classis Britannica, the fleet of the English Channel, declared himself Emperor of Britain and northern Gaul. His forces comprised his fleet, the three legions stationed in Britain and also a legion he had seized in Gaul, a number of foreign auxiliary units, a levy of Gaulish merchant ships, and barbarian mercenaries attracted by the prospect of booty. In 293 emperor Constantius Chlorus isolated Carausius by besieging the port of Gesoriacum (Boulogne-sur-Mer) and invaded Batavia in the Rhine delta, held by his Frankish allies, and reclaimed Gaul.