When Constantine died in 337, Constantius II led the massacre of his relatives descended from the second marriage of his grandfather Constantius Chlorus and Theodora, leaving himself, his older brother Constantine II, his younger brother Constans and two cousins (Gallus and his half-brother Julian) as the only surviving males related to Constantine. The three brothers divided the Roman Empire among them, according to their father's will. Constantine II received Britannia, Gaul and Hispania; Constans ruled Italia, Africa, and Illyricum; and Constantius ruled the East.
This division changed when Constantine II died in 340, trying to overthrow Constans in Italy, and Constans became sole ruler in the Western half of the empire. The division changed once more in 350 when Constans was killed in battle by forces loyal to the usurper Magnentius. Until this time, Constantius was preoccupied with fighting the Sassanid Empire, and he was forced to elevate his cousin Gallus to Caesar of the East to assist him, while he turned his attention to this usurper.
Constantius eventually met and crushed Magnentius in the Battle of Mursa Major, one of the bloodiest battles in Roman history, in 351. Magnentius committed suicide in 353, and Constantius soon after put his cousin Gallus to death. However, he still could not handle the military affairs of both the Eastern and German frontiers by himself, so in 355 he elevated his last remaining relative, Julian, to the rank of Caesar.
On 11 August 355, the magister militum Claudius Silvanus revolted in Gaul. Silvanus had surrendered to Constantius after the battle of Mursa Major. Constantius had made him magister militum in 353, with the purpose of blocking the German threats, a feat that Silvanus achieved by bribing the German tribes with the money he had collected. A plot organized by members of Constantius' court led the emperor to recall Silvanus. After Silvanus revolted, he received a letter by Constantius that recalled him to Milan, but which made no reference to the revolt. Ursicinus, who should have replaced Silvanus, bribed some troops, and Silvanus was killed.
Arbitio was the main general (magister militum) of Constantius. He was given a consulship in 355.
Jones also notes that Constantius "appears in the pages of Ammianus as a conscientious emperor but a vain and stupid man, an easy prey to flatterers. He was timid and suspicious, and interested persons could easily play on his fears for their own advantage. Ammianus also notes that Constantius II repeatedly murdered Roman aristocrats on the merest hint that they might be seeking the Empire themselves, including killing one because he had a dream about him, and another because he had a purple tablecloth which might have been fashioned into the robe of an Emperor.