Origin and career 
Silvanus was born in Gaul, the son of Bonitus, a Frankish general who had supported Constantine I against Licinius. In 351, he held the rank of tribune and was recorded as having defected to Emperor Constantius II at the Battle of Mursa Major, after initially supporting usurper Magnentius. Silvanus eventually rose to the rank of Magister militum: in 352-353, Constantius entrusted him with the task of driving the Germanic tribesmen raiding Gaul back beyond the Rhine, a task Silvanus fulfilled by bribing the Alamanni with the taxes he had collected, as well as curbing the bagaudae insurrection then flaring up again in central and northern Gaul.
Trial and usurpation 
Some of the courtiers of the Emperor Constantius managed to persuade him that Silvanus was planning to seize power. According to Ammianus, the praetorian prefect Lampadius and the ex-treasurer of the privy purse, Eusebius, used a sponge to alter a letter sent by Silvanus to his friends in Rome. The fake letter suggested that Silvanus was attempting to win support within the city for a coup. Constantius' court clique, with the exception of the Frankish generals Malarich and Mallobaudes, was uniformly against Silvanus. Courtiers Apodemius and Dynamius composed further fake letters. Constantius then held a trial where Silvanus' allies were successful in defeating the spurious charges against the general. Silvanus, unaware of the success of his supporters, responded to the threat of condemnation and execution by actually proclaiming himself emperor on 11 August 355 in Colonia Agrippina (modern Cologne). Late Roman historian Michael Kulikowski has argued that the entire episode was a later invention, created as an excuse to rid Constantius II of Silvanus before he became a threat. His primary basis of this argument is the fact that no coins minted with Silvanus' image have been found to date, since virtually every usurper minted coins as an attempt to legitimize his authority.
Death of Silvanus 
Constantius, who was in Milan, ordered Silvanus to come to him, and named Ursicinus to take over Silvanus' post. Ursicinus was himself at odds with Constantius' camarilla and Silvanus no doubt trusted the veteran general. The letter that Ursicinus gave to Silvanus did not indicate that Constantius already knew of Silvanus' bid for power, so Silvanus considered himself safe. However Ursicinus arranged the murder of Silvanus by co-opting some of the rebel soldiers. These men killed the usurper's guard and dragged Silvanus from the Christian church where he was worshipping and hacked him to death.
Ammianus's report of Silvanus's death 
It has been suggested by at least one scholar that Ammianus invented the entire coup attempt to gloss over the role played by his patron, Ursicinus, in the murder of a fellow general. This theory suggests that Constantius had grown suspicious of the popular Frankish general and so offered his post to Ursicinus, who then murdered his peer in the course of a botched change of command. It has been noted that Silvanus did not mint any coinage (which would have been a clear indication of a usurpation attempt), unlike other equally-short lived usurpers of the era, such as Poemenius. However, the thesis of a concocted coup attempt is generally rejected by scholars. The lack of numismatic evidence is not determinative, because Trier, the nearest minting centre to Colonia Agrippina, closed its gates to Silvanus.
Ammianus thus concludes his treatment of the Silvanus episode: "Such was the end of a commander of no small merit, who was driven by fear of the slanders in which a hostile clique had ensnared him in his absence to adopt extreme measures in self-defence" (15.5.29).
- Ammianus Marcellinus, History, 15.2.3.
- The argument that the usurpation was the product of a breakdown in communications between Constantius and his general in the field has been well argued by Nutt.
- See David Hunt's handling of Drinkwater's argument in "The Outsider Inside: Ammianus on the Rebellion of Silvanus" in Jan Willem Drijvers and David Hunt, eds., The Late Roman World and its Historian: Interpreting Ammianus Marcellinus (London, 1999).
- Syme; Matthews.
- The story of Silvanus short reign is told by Ammianus Marcellinus in his History, at 15.5, with further details scattered through the rest of his work
- Biography of Silvanus DiMaio, Michael, "Silvanus (355 A.D.)", De Imperatoribus Romanis
- Cameron, Averil and Garnsey, Peter (Eds.). (1998). The Cambridge Ancient History: Volume 13, the Late Empire, AD 337-425. Cambridge: Cambridge University Press. ISBN 0-521-30200-5
- Sir Ronald Syme, Ammianus and the Historia Augusta (Oxford: 1968).
- J.F. Matthews, The Roman Empire of Ammianus (London: 1989).
- T.D. Barnes, "Ammianus Marcellinus and the Representation of Historical Reality" (Ithaca: 1998).
- Jan Willem Drijvers and David Hunt, eds., The Late Roman World and its Historian: Interpreting Ammianus Marcellinus (London, 1999).
- D.C. Nutt, "Silvanus and the Emperor Constantius II" 7 9 (1973) Antichton 80-89.