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The Great Conspiracy was a year-long state of war and disorder that occurred in Roman Britain near the end of the Roman occupation of the island. The historian Ammianus Marcellinus described it as a barbarica conspiratio that capitalized on a depleted military force in the province brought about by Magnentius' losses at the Battle of Mursa Major after his unsuccessful bid to become emperor.
It is difficult to ascertain the exact chronology of the events because the main source—Ammianus—was living in Antioch at that time; thus his information looks second-hand and confused and, in addition, inconsistent with that produced by other sources. As a consequence there are several different views of what happened.
In the winter of 367, the Roman garrison on Hadrian's Wall rebelled, and allowed Picts from Caledonia to enter Britannia. Simultaneously, Attacotti, the Scotti from Hibernia, and Saxons from Germania landed in what might have been coordinated and pre-arranged waves on the island's mid-western and southeastern borders, respectively. Franks and Saxons also landed in northern Gaul.
These warbands managed to overwhelm nearly all of the loyal Roman outposts and settlements. The entire western and northern areas of Britannia were overwhelmed, the cities sacked and the civilian Romano-British murdered, raped, or enslaved.
Nectaridus, the comes maritime tractus (commanding general of the seacoast region), was killed and the Dux Britanniarum, Fullofaudes, was either besieged or captured and the remaining loyal army units staying garrisoned inside southeastern cities.
The miles areani or local Roman agents that provided intelligence on barbarian movements seem to have betrayed their paymasters for bribes, making the attacks completely unexpected. Deserting soldiers and escaped slaves roamed the countryside and turned to robbery to support themselves. Although the chaos was widespread and initially concerted, the aims of the rebels were simply personal enrichment and they worked as small bands rather than larger armies.
Historian Ian Hughes later argued that it is likely Nectaridus and Fullofaudes were killed by Saxon and Frankish raiders along the coast of Gaul, rather than by enemies in Britain, although Hughes's account lacks historical evidence.
Early unsuccessful attempts
The first was Severus, the emperor's comes domesticorum, soon recalled and replaced by Jovinus, the magister equitum.  Jovinus then wrote back to Valentinian requesting reinforcements. The Emperor recalled Jovinus—mostly likely to take part in a campaign along the Rhine, which was a higher priority—and then sent out Flavius Theodosius.
Historian Ian Hughes later argued that Severus and Jovinus were never actually sent to Britain, it being unlikely they would go all that way and come back. He proposed the following alternative chronology:
- June 367 - Valentinian informed of Saxon and Frankish raids along the coast of Gaul which resulted in the deaths of Nectaridus and Fullofaudes;
- Severus given a small force and ordered to gather information and counter the Saxon and Frankish raids;
- Valentinian moves to Amiens in order to gather intelligence and co-ordinate a response to the attacks;
- Severus returns with information that more troops are needed to restore order;
- Jovinus is ordered to the coast and begins repelling attackers;
- Jovinus passes word to the Emperor that Britain is under attack and he needs more troops to cross the Channel and restore the situation;
- Valentinian decides to assemble a force under Theodosius for the attack.
Arrival of Theodosius
In the spring of 368, a relief force commanded by Flavius Theodosius gathered at Bononia. It included four units, Batavi, Heruli, Iovii and Victores as well as his son, the later Emperor Theodosius I and probably the later usurper Magnus Maximus.
Theodosius took advantage of a break in the winter weather to cross the Channel to Richborough, leaving the rest of his troops at Bononia to await better weather. This enabled Theodosius to gather vital intelligence. He discovered that the British troops had either been overwhelmed, refused to fight or deserted; many also may not have been paid.
Once the troops landed, Theodosius marched with them to Londinium which he made his base. There he began to deal with the invaders:
There he divided his troops into many parts and attacked the predatory bands of the enemy, which were ranging about and were laden with heavy packs; quickly routing those who were driving along prisoners and cattle, he wrested from them the booty which the wretched tribute-paying people had lost. And when all this had been restored to them, except for a small part which was allotted to the wearied soldiers, he entered the city, which had previously been plunged into the greatest difficulties, but had been restored more quickly than rescue could have been expected, rejoicing and as if celebrating an ovation.
An amnesty was promised to deserters which enabled Theodosius to regarrison abandoned forts. A new Dux Britanniarum was appointed, Dulcitius, with Civilis granted vicarius status to head a new civilian administration.
After discovering that the local Areani had collaborated with the invaders, Theodosius ordered that this be wound up.
By the end of the year, the barbarians had been driven back to their homelands; the mutineers had been executed; Hadrian's Wall was retaken; and order returned to the diocese.
Considerable reorganization was undertaken in Britain, including the creation of a new province named Valentia, probably to better address the state of the far north. Claudian suggests that naval activity took place in northern Britain.
It is possible that Theodosius mounted punitive expeditions against the barbarians and extracted terms from them. Certainly, the Notitia Dignitatum later records four units of Attacotti serving Rome on the continent. The areani were removed from duty and the frontiers refortified with co-operation from border tribes such as the Votadini, marking the career of men such as Paternus.
Theodosius returned to Rome a hero, and was made senior military advisor to Valentinian I, replacing Jovinus. A decade later, his son became emperor. The Romans were able to end much of the chaos, though raids by all of the people listed above did continue.
Fictional accounts of the Great Conspiracy were featured in Wallace Breem's historical novel Eagle in the Snow, Stephen R. Lawhead's fantasy novel Taliesin, M. J. Trow's Britannia series and Jack Whyte's historical novel, The Skystone.
- Hughes, Ian (2013). Imperial Brothers: Valentinian, Valens and the Disaster at Adrianople. Pen & Sword Military. p. 56. ISBN 978-1848844179.
- "The Roman History of Ammianus Marcellinus". Bill Thayer's Web Site. Retrieved July 2015.
- Hughes p. 59.
- Hughes p. 58.
- Hughes p. 60-61.
- Hughes p. 69.
- Hughes p. 71.
- >Hughes p 85