Great Conspiracy

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.

The conspiracy

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.