In AD 270 Aurelian became emperor, the Gallic Empire collapsed and Britain was once more subsumed into the Roman Empire. Aurelian, however, lasted only five years before his assassination and a period of turmoil led to the murder of his successors Tacitus and Florianus. Probus assumed power in AD 276 and over the next six years succeeded in giving the empire some stability, though not in Britain, where there was rebellion. Probus sent Victorinus, a Moorish officer, to sort out this problem. Zosimus said that Victorinus put down the rebellion by a clever trick but unfortunately does not elaborate on this. Probus also used Britain as a place to send captives including Burgundian and Vandal prisoners, intending that they should settle in the province; whether the intention was to get rid of them or, as Zosimus said, to use them to assist in putting down the rebellion is not clear. Probus lost the empire to Carus, his Praetorian Prefect, in AD 282 and Carus’s son Carinus took charge of the western part of the empire, assuming the titles of Britannicus Maximus and Germanicus Maximus.

Stability was not restored to the empire until Diocletian secured the imperial throne in AD 284. The next year he appointed Maximian as Caesar, his deputy with control over the western part of the empire. A year later, because of his successful rule in Gaul, Maximian was elevated to the position of joint emperor. A signet ring found in Britain portrayed their relationship. Diocletian was represented as Jupiter making decisions while Maximian was Hercules roaming the world to protect the empire against the forces of destruction. As Maximian had married Diocletian’s daughter this also indicated the link with Hercules as a son of Jupiter.

Maximian’s main task was to prevent pirate raids, which were increasingly occurring along the coasts of Britain and Gaul. This brought him into conflict with M. Aurelius Mausaeus Carausius, a Menapian from a region within modern Belgium. With his command covering the coasts of Belgica and Armorica and probably Britain, he was in charge of a fleet to drive off the pirates, mainly Franks and Saxons.

From his base at Boulogne, Carausius was successful in putting down piracy, so much so that he made himself a wealthy man. He was accused of letting pirates raid the coast, then capturing them when they left, and seizing their booty. This did not fit in with Maximian’s scheme of government for the west. He put a price on Carausius’s head whereupon Carausius, in either AD 286 or 287, sailed for Britain and declared himself emperor. Britain was now a province worth governing, as it was one of the most secure, prosperous and relatively peaceful in the empire. Why, therefore, it supported Carausius is not clear. He may have secured enough wealth to pay subsidies to troops to bring them over to his side or the troops in Britain may have become alienated towards, or indifferent to, the present imperial government. Carausius might have been an officer in a Roman legion for part of his career and was therefore known to the troops. Civilians, such as wealthy villa owners or merchants, may have resented paying taxes to Rome and taken an opportunity to rid themselves of this burden.

Maximian could not move against Carausius immediately as he was concerned with suppressing rebellions on the Rhine frontier. By 289, however, he had gathered a fleet but Carausius’s sea tactics ensured that he drove off the Roman fleet and Maximian appears to have been unable to secure a decisive victory.

Instead, he put the best face he could on the matter and gave Carausius the title of Augustus. How far Carausius’s empire extended is uncertain. He had control of Britain and possibly the coastal areas of Gaul – coinage found in northern Gaul has his name stamped on it. The coins had been struck at several mints, one being in London and another at Rouen. He needed this coinage to pay his troops but the coins also show his imperial expectations. Early ones depict him clasping hands with a figure of Britannia and the legend ‘Expectate veni’ (Come, we have expected you). Others legitimate his authority with the words ‘Pax AUGGG’ (Peace of the three emperors).

Carausius’s empire was not destined to last. He was assassinated by Allectus, his financial officer, whose plot against him succeeded in AD 293, probably because Carausius had been weakened when Constantius, elevated by Maximian to the post of Caesar, seized Boulogne. This had loosened Carausius’s grip on the coast of Gaul. Constantius was now determined to seize back control in Britain, but he did not organize an attack until AD 296, possibly finding it difficult to raise sufficient ships until then. He divided his fleet into two separate parts. One, led by Constantius himself, sailed from Boulogne towards the Thames estuary; the other, under the command of the Praetorian prefect, Asclepiodotus, sailed from the Seine estuary. He was extremely lucky for a thick sea mist enabled him to avoid been seen by Allectus’s fleet, which was stationed off the Isle of Wight. He landed somewhere in the region of the Solent and, once he had embarked, ordered his men to burn the boats. He may have wished to prevent the enemy getting hold of them or perhaps he did not have enough men to leave some to guard the ships.

Burning the boats also ensured that his men knew there was no way back.

The Roman army in the time of Constantine the Great.

Constantius’s fleet had been delayed by bad weather and only a few ships got through to land troops near Richborough. The greater danger, however, came from the troops of Asclepiodotus, who was advancing towards London. Allectus may not have been with his fleet but was somewhere in the south-east, for he was able to raise an army to meet the oncoming forces. The two armies met either in Hampshire or West Sussex. The battle resulted in the total defeat of Allectus and his subsequent death. The remnants of his forces retreated towards London, only to be trapped by those soldiers who had managed to land from Constantius’s ships. Allectus’s forces included Frankish soldiers who sacked the city before probably intending to return to their homes across the North Sea. They were slaughtered without mercy.

Constantius quickly sailed for Britain and was able to enter London in triumph. A bronze medallion, struck at Trier, records his entry. On one side is the head of Constantius with his titles. The other shows him riding into a walled city identified by the letters ‘LON’ for Londinium underneath. Before the wall kneels a figure with arms raised in supplication at his entry. His fleet, represented by a boat, has obviously sailed up the Thames. The inscription Redditor Lucis Aeternae (Restorer of the light that burns for ever) indicates the gratitude of the citizens.

Constantius immediately focused his attention on ensuring control over the army as well as securing the defences of the province, for Irish pirates were now menacing the west coast, the Picts were restless in the north and there were also pirate attacks on the eastern seaboard. A number of forts had already been constructed on the south-east and south coasts of Britain and these were reconditioned. These have been called the Saxon Shore forts but they were not built at the same time to form one coordinated defensive structure. Brancaster (Norfolk) and Reculver were built about AD 230. To these were added Burgh Castle (Norfolk), Walton Castle (Suffolk), Bradwell (Essex) and Lympne (Kent) in the 270s. At that time the small fort of Richborough was given more defensive walls and Dover was refortified. Carausius may have added Portchester but the fort of Pevensey (East Sussex) was not built until AD 370.

These were not forts in the strict military sense of housing units of troops trained to attack. Their stout walls and formidable turrets seem to have been designed for defence. Ballistae and catapults could be placed on these turrets. Soldiers and their families, who normally lived outside the forts, could take shelter in them.

Their defences were against raiders coming across the North Sea, drawn by the wealth of the province. Yet raiders could easily slip between the forts and land anywhere along the undefended parts of the coast so it would have been necessary to construct a unified system of command that would ensure the defence of the whole eastern coast and its hinterland. It is also possible that a line of watchtowers was constructed along the Thames estuary, as one has been excavated at Shadwell. London would have been a great prize for seaborne raiders and early warning of their coming was essential. The western coastline was not left unprotected. Forts were built to guard the estuaries from Cardiff to Lancaster from raiders coming from Ireland and Scotland.

All these forts would have had to be used by the fleet. The Classis Britannica (the British fleet) already had bases at Dover and Richborough and after Carausius’s rule the latter became its chief base. Gaius Aufidius Pantera, prefect of the fleet, dedicated an altar to Neptune at Lympne, which suggests it was another base. Other forts would have provided facilities to moor and repair ships. It would have been easy to link the forts by sea and arrange patrols along the coast. There is no evidence of a road system linking one with another. In the south roads lead from Richborough, Dover, Reculver and Lympne to Canterbury, which has been suggested as being an off-duty station for the troops.

The northern frontier required extensive restoration, not because of destruction, but from decay. There had been no programme of repairs since Severus’s reign. It was time for intensive refurbishment and this was continued beyond Constantius’s visit. Some forts had their garrisons reduced; at Wallsend and Chesters buildings were demolished in one area of the fort and not rebuilt. In other forts, buildings were abandoned. It has been suggested that these open spaces were to house the tents of mobile forces that could move out quickly to put down uprisings. Constantius appeared to have ordered forts to reduce in size their principal buildings, as happened at Great Chesters and Housesteads. At Birdoswald the principia (headquarters), the praetorium (commander’s residence) and the baths needed extensive repair.

There were also differences in how the troops were housed. At some forts, such as Housesteads, rather than restore the barrack blocks, ‘chalets’ were built, laid out in rows, as if indicating that soldiers and their families could live there.

Alternatively these may have housed the exploratores (scouts) whose duties took them into the Scottish lowlands. Restoration was also made to the south of Hadrian’s Wall at Lanchester, Binchester, Malton and Ilkley. In addition the walls of the fortress of York were rebuilt and the riverfront enhanced with a series of multangular towers, both for protection and to demonstrate the power of military might. This restoration took place over a long period and extended into the fourth century.

Constantius returned to Gaul in AD 297 to concentrate on the problems there. He took skilled men from Britain to rebuild part of the city of Autun in France, which may indicate that the towns in Britain had not suffered during Carausius’s rule. A panegyric on Constantius Caesar, compiled in AD 297, recorded that the Britons had greeted him, ‘with great joy after so many years of most wretched captivity …

At last they were free, at last Romans, at last restored afresh by the true life of the empire.’ Allowing for the hyperbole it would seem that a new era had begun in Britain but to ensure that it would continue to be a prosperous province its defences had to be secured. That all was not yet secure is indicated by the return of Constantius in AD 305 to sort out further problems in the north.