In AD 286 Maximian, newly appointed as his co-emperor by Diocletian, was in Gaul (modern day France) suppressing a revolt by runaway slaves and peasants known as the Bacaudae. At this time the south-eastern coast of Britain and northern Gaul were being subjected to raids by Saxon pirates and it was thought necessary to create a naval force to deal with them.

Command of this fleet was given to one of Maximian's lieutenants called Carausius, who had already demonstrated his skill and valour. Soon after his appointment, however, complaints were made that instead of returning any recaptured booty, Carausius was expropriating it for his own use. Maximian ordered his arrest and execution but Carausius forestalled this by sailing off to Britain and declaring himself emperor. How this was accomplished is unknown and the literary evidence for the chronology and events of this rebellion are extremely scanty. The main sources are two panegyrics, one in honour of Maximian, delivered by Claudius Mamertinus in AD 289, and the other by Eumenius in AD 297 for Constantius I. There are also sketchy accounts by Aurelius Victor and Eutropius over half a century later, the ramblings of Geoffrey of Monmouth written circa AD 1136, reputedly based on Welsh folklore, and the medieval Scottish Chronicles of John of Fordun and Hector Boethius. Although writing a thousand years after the event, the Chroniclers add many details not found elsewhere, such as a supposed alliance with the Picts and Scots which enabled Carausius to defeat the Roman garrison and take control of the island.

In general they are in agreement, that Carausius first sailed round Britain and then, after landing in the north, defeated the Roman governor, Quintus Bassianus, in a battle fought near York. So little is known about Carausius that were it not for the famous Carlisle milestone we would not even be aware of his full name. This stone, discovered in 1875, bears the legend IMP C M AVR MAVS CARAVSIO INVICTO AVG. It had been reversed in the ground and re-used in the time of Constantius I. His name and titles were therefore Imperator Caesar Marcus Aurelius Mausaeus Carausius the Invincible (unconquered) Augustus (emperor). According to the historians he was a citizen of Menapia, part of modern Belgium, and stress that he was "vilissime natus" - of the most humble birth.

In addition to Britain, Carausius must have controlled part of northern France, because it was necessary for Constantius I, who was made Caesar of the Western provinces on 1 March AD 293 and given the immediate task of recovering Britain, first to capture the port of Gesoriacum (Boulogne). This he accomplished by building a mole across the entrance to the harbour and preventing supplies and reinforcements from being sent by Carausius. In the wave of panic that followed the loss of Gesoriacum the enemies of Carausius assassinated him and apppointed his chief minister, Allectus, emperor in his place. Meanwhile Constantius secured the rest of Gaul and made his preparations for an invasion.

The prime obstacle facing him was the "Classis Britannia", the British Fleet, already enjoying a fearsome reputation for its defeat of a previous invasion attempt by Maximian in AD 289 (explained away by Roman historians as the result of an "inclementia maris" - an inclement sea). Robbed of its main base at Gesoriacum it now fell back on Clausentum (Bitterne in Southampton Water). In addition to the fleet was the series of forts guarding all the navigable estuaries from Portchester, near Portsmouth in Hampshire, to the Wash, known as the "Litus Saxonicum", the Saxon Shore. Some of these forts were built in places with a long history, for example Richborough in Kent, while others were completely new sites, such as Pevensey in Sussex. All date from the latter part of the 3rd century and may either have been built by Carausius himself or were part of the general defensive trend inaugurated in the time of Aurelian (AD 270-275), who ordered walls to be built round Rome.

By AD 296 Constantius had assembled two large fleets, one under his own command at Gesoriacum, the other under his Praetorian Prefect, Asclepiodotus, at Rouen. A false rumour caused this latter force to set sail and, when he heard this, Constantius hurried after it. Thanks to a thick fog Aslcepiodotus managed to avoid the British Fleet and landed near Southampton, burned his boats and marched for London. On hearing the news Allectus gathered his army and hurried to meet them, but was defeated and killed. The scattered remnant of his army fled back to London but were prevented from sacking it by the belated arrival there of Constantius.

This event was celebrated by a large gold medallion showing Constantius entering the gates of London and the legend REDDITOR LVCIS AETERNAE (Restoration of Eternal Light).

From http://www.users.globalnet.co.uk/~kenelks/carausius.htm

LVF