Great Conspiracy

The Great Conspiracy was a year-long state of war and disorder that occurred near the end of Roman rule in Britain. Fourth-century Roman historian Ammianus Marcellinus describes it as a barbarica conspiratio ('barbarian conspiracy') which took advantage of a depleted military force in the province; many soldiers had marched away with General Magnentius in his unsuccessful bid to become emperor. Few returned, and supply, pay, and discipline in the following years may have been deficient.

The precise chronology of events remains unclear. Ammianus, although being the author of the most comprehensive and widely-cited surviving sources on the Conspiracy, was living in Antioch during the time that it took place. His account is thus likely to be derived from other sources and exhibits inconsistencies with other known documentation. The nature and the causes of the events is therefore not entirely clear. While it has been posited that this period of unrest correlates, or was even caused by, extraordinary droughts in Britain in preceding years, this link seems tenuous and is not supported by historical evidence.