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The Great Conspiracy is a term given to a year-long war 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 of the Battle of Mursa Major after his unsuccessful bid to become emperor.
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 coordinated and pre-arranged waves on the island's mid-western and south-eastern 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 Count of the Saxon Shore, was killed and the Dux Britanniarum, Fullofaudes, was either besieged or captured, the remaining loyal army units staying garrisoned inside southeastern cities. The miles arcani 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.
Emperor Valentinian I was campaigning against the Alamanni at the time and unable to respond personally. A series of commanders to act in his stead were chosen but swiftly recalled. The first was Severus, the emperor's comes domesticorum, soon replaced by Jovinus, the magister equitum; rumours of disasters dogged them, however, and it took almost 15 months before a capable replacement was sent.
In the spring of 368, a relief force commanded by Count Theodosius arrived in Britannia from Gaul. He brought with him four units, Batavi, Heruli, Iovii and Victores as well as his son, the later Emperor Theodosius I and probably the later usurper Magnus Maximus. He marched from Richborough to Londinium and began to deal with the invaders. 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.
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. 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, and Jack Whyte's historical novel, The Skystone.