Pannonia, Diocese of

Constantine prevails again against his Eastern counterpart Licinius (shortly after he had named as co-emperor Valerius Valens, who had previously been dux limitis in Dacia) in the plain of Mardia (also known as Campus Ardiensis) in Thrace.

The emperors are reconciled after this battle.

Licinius according to a peace signed on March 1, 317, has to execute Valerius Valens, and cedes territory to Constantine, including …

…the provinces of Pannonia and Moesia.

Magnentius moves toward the Danube, where he allies himself with the commander of the Danubian troops, Vetranio, who at the behest of Constantia, the sister of Constantius II, had stood successfully for election by his troops as Augustus on March 1, 350 and taken power in Illyricum.

Constantia’s brother Constans had been killed by Magnentius earlier this year and she probably thinks Vetranio can protect her family and herself against the usurper.

Vetranio accepts and coins are minted in his name, showing the title of Augustus (full emperor), rather than Caesar.

This revolt has a loyalist mark, since Constantina supports Vetranio and Constantius II himself had recognized him, sending Vetranio the imperial diadem as well as money to raise an army.

This arrangement ends quickly with the abrupt overthrow of Vetranio by Constantius, who has broken off his war in Syria with Persia, and marched west.

Despite Magnentius’s effort to gain Vetranio to his cause, the old general reaches Constantius with his army, first meeting with Constantius at Serdica; …

…the combined forces then move on to Naissus in Pannonia.

On December 25, 350, both men mount a platform before the assembled troops; Constantius manages, by means of a strong speech, to have the soldiers acclaim him emperor.

He then takes the purple away from Vetranio, leads the old man down the stairs of the platform, calls him father, and escorts him to the dinner table. (Constantius will allow Vetranio to live as a private citizen at Prusa on the equivalent of a state pension for six years until his death.)

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.

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.

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.