Barbatio, a soldier of unknown origin, began his rise when he was appointed to command the household troops of Caesar Gallus, a cousin of Constantius II.
For his part in the affair, Barbatio was awarded by Constantius with a series of promotions, making him commander of the infantry in Gaul after the death of Claudius Silvanus in 355.
Not long after the death of Gallus, Constantius summoned Julian, the dead man's half-brother, from his studies in Athens to the royal court in Milan.
It was intended that two armies, the first commanded by Julian and the second by Barbatio, would advance in a classic Roman tactic known as a forceps or forfex, forming diverging wings, embracing and destroying the enemy.
Eventually the planned pincer movement was frustrated when Barbatio, in the words of Ammianus, "…as if he had ended the campaign successfully, distributed his soldiers in winter quarters and returned to the Emperor's court to frame some charges against Caesar, as was his custom.
Gibbon wrote in The History of the Decline and Fall of the Roman Empire, "But the hopes of the campaign were defeated by the incapacity, or the enemy, or the secret instructions of Barbatio; who acted more as if he had been an enemy of the Caesar and a secret ally of the Barbarians"[8] In 359, with Barbatio away on another campaign, his wife, Assyria, whom Ammanius describes as an "indiscreet and silly woman", decided to write to him, seemingly fearful that he was about to cast her off.
Her letter, which has not survived, hinted, in Ammianus' account, at Barbatio's own imperial ambitions, and his possible intention of marrying the empress Eusebia in the event of Constantius' death.