Julius Constantius' second wife, Basilina, had died soon after the birth of Gallus' half brother Julian, who was thus early left an orphan.

With Gallus, seven years his senior, he had been brought up in obscurity, first by Eusebius, Arian bishop of Nicomedia in Bithynia, and later at the remote estate of Macellum in Cappadocia.

By the patronage of Eusebia, wife of Constantius II, Julian, at age nineteen, had been allowed to continue his education, first at Como and later in Greece.

Having developed a fondness for Hellenic literature, he secretly converts in 351 to the pagan Neoplatonism, recently “reformed” by the late Syrian philosopher Iamblichus, and is initiated into theurgy by Maximus of Ephesus, the Neoplatonist philosopher and theurgic magician whose most spectacular achievement has been the animation of a statue of Hecate.

The domed, circular Mausoleum of Santa Costanza, constructed in Rome around 350 to house the remains of one of Constantine’s daughters, is an early example of the centralized sanctuary type of ecclesiastical architecture.

An alternative to the longitudinal basilica, the more diverse centralized type, which could be built to a circular, polygonal, square, or Greek-cross plan, could serve as a baptistery, a memorial, a martyrium, a mausoleum, or as a congregational church.

Extensive mosaics decorate the ambulatory vaults of the Mausoleum of Santa Costanza with geometric and figurative patterns.

Among these are a magnificent version of the popular Hellenistic motif called the "unswept floor"—a scattering of shells, bowls, pitchers, branches, and birds that might have covered a floor after a feast.

The mausoleum’s dome is adorned with Old and New Testament scenes (now lost).

Magnentius Proclaims Magnus Decentius as Caesar (Winter 350/351 CE)

Following his usurpation of the Western Roman Empire in 350 CE, Magnentius faced the looming threat of Constantius II, who ruled the Eastern Roman Empire and sought to avenge the murder of his brother, Constans. To strengthen his rule and secure the northern frontier, Magnentius proclaimed Magnus Decentius, likely his brother, as Caesar, probably during the winter of 350/351 CE.

1. The Role of Magnus Decentius

• Decentius was entrusted with the defense of Gaul and the Rhine frontier, a critical region vulnerable to Germanic invasions.
• By appointing a Caesar, Magnentius sought to consolidate his power in the West, allowing him to focus on fighting Constantius II in the East.
• The move also served to legitimize his rule, mirroring the Tetrarchic system established under Diocletian, where emperors had junior co-rulers.

2. The Struggle Against Constantius II

• Despite these strategic moves, Magnentius suffered a crippling defeat at the Battle of Mursa (351 CE) against Constantius II, greatly weakening his position.
• As Magnentius’ power declined, so too did Decentius’ authority in Gaul.
• By 353 CE, after Constantius' final victory, both Magnentius and Decentius faced collapse.

3. The End of the Usurpers (353 CE)

• Magnentius, seeing no path to victory, committed suicide in August 353 CE.
• Decentius, realizing his position was hopeless, also took his own life shortly after.
• With their deaths, Constantius II became the sole ruler of the Roman Empire, ending the last major civil war of the mid-4th century.

4. Conclusion: A Short-Lived Attempt at Stability

• The appointment of Magnus Decentius as Caesar was a practical move to protect Gaul and the Rhine frontier, but it ultimately failed to stop Constantius II's advance.
• Their defeat marked the reunification of the empire under Constantius II, but the losses suffered in the civil war severely weakened the Western Roman military, making it more vulnerable to future external invasions.

Magnentius’ attempt to maintain a breakaway empire ultimately collapsed, demonstrating the chronic instability of the late Roman Empire, where military usurpations frequently led to destructive civil wars.

Magnentius prepares for the war with Constantius by stripping the borders of Gaul of their defenses and seeking aid from the Germans on the Rhine frontier.

Emperor Constantius II, like his father before him, shows a preference for the Christian religion, which he favors over all others, including Judaism.

Unlike his father, however, Constantius allows Christians to persecute the pagans and the Jews.

Christian clergy practices intolerance toward non-Christians, both through the secular arm and directing the angry crowds, which attack and destroy synagogues and temples.

The Jews eventually decide to react, opposing Christian proselytism and showing intolerance toward Jewish converts.

The fiery sermons, preached in synagogues against Edom, are in fact directed against those Romans who, after removing the Jews' political independence, are now repressing their religion.

Constantius in 350 is engaged in a campaign in the East against the Sassanids.

He is however forced to return to the West to counter the usurpation of Magnentius, who had murdered Constantius' brother and colleague, Constans.

Constantius therefore decides to appoint his cousin Constantius Gallus Caesar of the East.

Constantius, at Sirmium on March 15, 351, proclaims Gallus his coadjutor and eventual successor to handle problems in the east, and arranges Gallus's marriage to his sister Constantia.

Gallus, the second of three children of Julius Constantius, had escaped the slaughter that followed the death of Constantine in 337—his elder brother had been killed in 34—purportedly because he was suffering at the time from what was thought to be a fatal illness.

(Julian, the youngest brother, will mention years later that he and Gallus were told repeatedly that Constantius' actions had been the result of a combination of misinformation and pressure from the soldiery, and that the emperor had come to regret his actions, which, in his mind, he linked to his childlessness and to military misfortunes on the Persian front.)

Magnentius is able to successfully ambush the emperor's forces near Adrana (Atrans) in the spring, and advance into the province of Pannonia Inferior.

Constantius Gallus, a son of Julius Constantius by his first wife Galla, was born in Massa Veternensis, Italia, after his father had been recalled from exile.

Gallus' paternal grandparents were the Western Roman Emperor Constantius Chlorus and his second wife Flavia Maximiana Theodora.

Julius Constantius was a paternal half-brother of the Roman Emperor Constantine I, which, in turn, means Gallus is a half-first cousin of Constantine's sons, Constantine II, Constantius II and Constans.

At some point prior to 331/2, Gallus' mother, Galla, seems to have died, as at that stage, Gallus' father was married to one Basilina and had had a son by that marriage.

Gallus has three siblings: an elder sister, of unknown name, who was the first wife of Constantius II, an elder brother, also of unknown name, who died in the purges that occurred in the imperial family after the death of Constantine I in 337, and a younger half-brother by his father's second marriage, named Flavius Claudius Iulianus, commonly known as Julian.

Gallus himself is one of the only imperial males, outside of the three sons of Constantine I and Fausta, who were not killed; the others being Gallus' younger half-brother, Julian, and their cousin, Nepotian, each of whom was very young at the time.

Gallus was said by the historian Ammianus Marcellinus to have been very good looking, making particular note of his soft blond hair.

There is debate over where Gallus spent his youth.

One view is that he lived with Julian in Nicomedia under the care of Eusebius, who was the bishop of Nicomedia until 340, at which point, Eusebius was made bishop of Constantinople, which necessitated Gallus and Julian to follow him to that city.

It is believed that after the death of Eusebius in 341, Constantius then sent Gallus and Julian to continue their studies at the imperial household in Macellum, Cappadocia.

An alternative view claims that hints in the sources suggest that Gallus was sent to Ephesus to study, then to a type of exile in Tralles and from there to the imperial household in Macellum.

In 350, Magnentius, the commander of the Herculians and Iovians, the Imperial guard units, had rebelled and killed the emperor Constans, claiming the purple.

Constantius II prepares to move against the usurper, but needs a representative in the East, so he calls Gallus at Sirmium, raises him to the rank of caesar (15 March 351), gives him the name Constantius, and strengthens the bonds with his cousin by allowing Gallus to marry his sister Constantina.

Gallus and Constantina, who probably shares her brother's aim of controlling the young Caesar, set up residence in Antioch, arriving on May 7 of 351.

During the period between the passage of Constantius in the West and the arrival of Gallus in the East, or immediately after the arrival of the Caesar in Antioch, the Jews revolt in Palestine.

The usurper Magnentius, refusing Constantius' offer of compromise and suffering setbacks at Siscia (modern Sisak, Croatia) and Sirmium (modern Sremska Mitrovica, Serbia), pursues Constantius to …

…Mursa (modern Osijek, Croatia), where on September 28, 351 emperor and usurper meet in battle.

Constantius before the battle had set Flavius Philippus, his Eastern Praetorian prefect, to negotiate with Magnentius, requiring the usurper withdrawal to Gaul.

After this inconclusive mission, one of Magnentius’s commanders, Silvanus, and his men had deserted to Constantius.

Magnentius leads his troops into combat; Constantius leaves the battlefield to preach on the nearby tomb of a martyr.

Constantius' army is outnumbered, but after prolonged fighting his cavalry routs Magnentius' right wing, and soon his victory is complete.

The battle entails major losses on both sides, leaving the two strongest armies of the empire—those of Gaul and of the Danube—massacred, thus severely crippling the military strength of the Roman Empire and compromising its defense.

Known as the bloodiest battle of the century, it is also the first defeat of Roman legionaries by heavy cavalry.

Losses suffered by the victors (thirty thousand) exceed those of the routed force (twenty-four thousand), however.

Magnentius flees the field and retreats to northern Italy to regroup his forces.