Silvanus was born in Gaul, the son of Bonitus, a Laetic Frankish general who had supported Constantine I in the civil war against Licinius.
In AD 351, he held the rank of tribune in the army of the rebel Magnentius, and in this role he defected to Emperor Constantius II at the Battle of Mursa Major.
[2][3] Gaul had been subject to raiding and looting by Alemanni tribesmen; Constantius entrusted Silvanus with the difficult task of driving the invaders back beyond the Rhine, and restoring the fast-eroding Roman authority in the province.
Silvanus fulfilled his mission through a combination of military action and bribing the Alemannic chieftains with the taxes he had collected, and also suppressed the local bagaudae insurrections flaring up again in central and northern Gaul.
But Malarichus, a Frank like Silvanus and the man who would later be offered a position in the military magisterium but would turn it down, and who was at this time a tribunus in the imperial court, spoke up to defend the suspected magister militum.
We should remember that Ammianus says Arbitio convinced Constantius to send Silvanus to deal with the raids along the Rhine frontier in the hopes he would be too far removed from court to exert any influence, and would maybe even perish.
He had been receiving word from his friends about the emperor’s suspicions, and that Apodemius had been seizing his properties in Gaul with impunity, and he did not feel confident that his innocence would protect him.
Silvanus, seeing no other viable choice, decided he would truly have to usurp imperial power, and after discussions with the chief officers of his army, he declared himself emperor on 11 August 355 in Colonia Agrippina (modern Cologne).
Hunt further argues that the rebellion was simply smaller than Ammianus would have us believe, and Silvanus had not yet secured the mint at Trier to begin distributing coins.
Furtive connections are established between the usurper and other important individuals and groups: military leaders and soldiers, senators, the Church, and civilian bureaucrats, including those who ran the imperial fabricae and mints.
This could in fact indicate that on that date Silvanus had still not decided on his course of action, and thus potentially only had three or fewer days to prepare, assuredly not enough time to connect with the commanders in charge of the mints in Trier.
When news of Silvanus' rebellion came to the imperial court at Milan, Ammianus reports that at first there was great panic, and Constantius called a midnight meeting of his consistorium.
[13] 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.
[14] 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.
Ammianus 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.