Late Roman Empire, Diocletian, Maximian: Britannic Empire (286-296)

While Maximian was defeating the bagaudae (286), the Frankish and Saxon pirates had been threatening the coasts of Britain and Gaul, and the mouth of the Rhine. To defend these coasts Maximian appointed ²⁶Aurelius Mausaeus Valerius Carausius (fl.286-293), a Menapian (a Belgic tribe), to command a fleet based at Gesoriacum (Boulogne). Carausius crushed the pirates but then used his naval power to occupy Britain and declare himself emperor in Britain and northern Gaul (Imperium Britanniarum). For the time being Maximian accepted the situation as he was dealing with invasions by the Alamanni and Burgundians.

This situation continued until 293, when ¹⁵Flavius Valerius Constantius (c.56; fl.294-306) blockaded Gesoriacum and built a giant mole to block the harbour. Carausius failed to relieve the city and retreated back to Britain. The port surrendered and with his position having weakened, Carausius was assassinated by his treasurer Allectus. Constantius spent the next three years assembling a fleet and campaigning against Allectus’ Frankish allies near the mouth of the Rhine. 

In September 296 Constantius launched an invasion to recover Britain. One arm of his force, led by the praetorian prefect ⁴⁵Julius Asclepiodotus (fl.292-296), sailed up the Solent Strait and landed near Clausentum (=Southampton).

Allectus advanced to meet Asclepiodotus and in a decisive battle, fought somewhere near Calleva Atrebatum (=Silchester), Allectus was defeated and killed. During this time, Constantius leading a second army had crossed the Channel and arrived at London just in time to intercept and kill the remnants of Allectus’ troops fleeing from the battle. 

In 298, Constantius fought in the Battle of Langres (northeast France) against the Alamanni, and defeated them again at Vindonissa (Windisch, Switzerland), thereby strengthening the defences of the Rhine frontier.