According to the Roman historian Ammianus Marcellinus, an event called barbarica conspiratio (“barbarian conspiracy”) took place in the second half of the 4th century CE, which involved a simultaneous attack on Roman Britain by various barbarian tribes. It is not clear whether this was an organized action; however, it certainly severely damaged the province, which was abandoned by the Romans half a century later.
It is difficult to clearly establish the date of this event, because our only source is Ammianus, who was in the eastern parts of the Roman Empire, and his knowledge was rather second-hand. Moreover, he wrote about these events almost 20 years later. It is certain that the “conspiracy” could not have taken place before 367, and it certainly had during the reign of Emperor Valentinian I (364-375 CE). The ruler personally waged battles against the Alamans on the lower Rhine in the winter of 367 CE. Probably during his trip from Amiens to Treves (Trier), he heard the alarming news from Britain, where the population was revolting, oppressed by dishonest officials and inept administration, and desertion was spreading in the military branches. He died, among others his commander Nektaridus (comes maritime tractus), and Fullofaudes (DuxBritanniarum) was taken prisoner.
According to the official version of historians, in the winter of 367 CE, the Roman fort at Hadrian’s Wall (the northern border of Britain) revolted and let Picts (from today’s Scotland) to areas under Roman control. Moreover, from what is now Ireland, the Scoti and Attacotti peoples and the Saxon Saxons attacked the island from the west and south; a wave of barbarians also landed in northern Gaul. On top of everything else, a man named Valentinus, an expelled citizen of Pannonia, was acting hostile in Britain, along with his group.
The sudden invasion of unprepared and weak Roman forces caused the western and northern parts of the island to succumb to the onslaught of warriors: many Roman citizens were captured or killed. Cities were conquered, plundered and destroyed. Roman intelligence services (arcani), instead of gathering information about attacks by barbarians, probably bribed, disinformed their superiors. In addition, the situation was worsened by the fact that Britain was in chaos, and the lack of a strong command caused desertion among soldiers. Moreover, slaves abandoned their masters and plundered property.
The originally Roman expedition against the invaders was to be led by Severus (comes domesticorum), who was later replaced by Jovinus (magister equitum). However, Jovinus was also dismissed (perhaps due to awkwardness) and the campaign was eventually led by Flavius Theodosius – the father of the later emperor Theodosius I – who came from Spain. British historian Ian Hughes believes that in the end the action against the barbarians in Britain was not led by Severus or Jovinus, and only by Theodosius. Probably Severus and Jovinus commanded the operation to recapture the lands in northern Gaul, which were attacked from the coast by the Saxons and the Franks.
In the spring of 368 CE, Flavius Theodosius gathered troops at Bononia (the present French coastal city, Boulogne). They included the following divisions: Batavi, Heruli, Iovii and Victores. With Flavius Theodosius was his son, also Theodosius, and Magnus Maximus, who later became usurper in Britain.
The commander and some of the soldiers landed in Rutupiae (present-day Richborough on the south coast of England). When the remaining troops landed on the shores of Britain, he marched to Londinium, setting up his base of operations there. In the meantime, he learned that Roman soldiers in Britain were mostly undisciplined and received no wages.
According to Ammianus Marcellinus, Theodosius divided his army into several parts and began the process of liquidating groups of barbarians, collecting their loot and liberating prisoners. Roman troops also recaptured numerous cities and fortresses; defenders, often rebellious soldiers, opened the gates after having been assured of amnesty. The effectiveness of the offensive and the actions of Theodosius’ army was really impressive.
At the end of the year, the barbarians left Britain, and the Romans only had to clean up the provinces. Theodosius appointed a new Dux Britanniarum – in chief in Britain – Dulcitius; and the administration administrator of (vicarius) Civilis. After an investigation, it turned out that areani were acting to the detriment of the state, and the staff was replaced. The rebels on the Roman side were caught and sentenced to death. In the north of Britain, a new province of Valentine was created to better counter the Pictish invasions. The borders were strengthened and the damaged fortifications were rebuilt.
It took Theodosius two years to get the situation under control, expel the barbarians, re-fortify the border at Hadrian’s Wall, reform the administration and fortify the coast. Upon his return, in recognition of his merits, the emperor appointed him the head of the cavalry, and for many, he became a hero.
The events discussed were only the beginning of the “dark days” to come to Britain. Over time, the multitude of problems faced by the Western Roman Empire (from 395 CE) on the European continent will lead to a situation that any requests for support from Britain will remain unanswered by the emperor. This will eventually lead to the weakening of the province and the decision to leave the province by the Romans at the beginning of the 5th century CE.