"Let Us Have No Mock-Emperors!" ~ The Battle of Mursa Major, AD 351

The portly profile of Magnentius may be seen on this
bronze follis from AD 352.

On this date in AD 351, the armies of the Western usurper Magnus Magnentius suffered a major defeat at the battle of Mursa Major in the Roman province of Pannonia—present day Croatia. Magnentius had decided to risk battle with the superior forces of Constantius II, the sole remaining son of Constantine the Great. This bloody conflict—which pitted tens of thousands of veteran Roman soldiers against each other—would have a profound negative impact on manpower resources of the Empire in the years ahead.

After the death of Constantine the Great in AD 337, the Roman Empire was divided among his three sons: Constantine II, Constans, and Constantius II. After a few years, this status quo proved untenable. As the elder brother, Constantine II felt that his share of the empire was insufficient, and began making demands on the youngest, Constans, who ruled Italy, Africa and Illyricum. For three years, Constans appeared to concede to his domineering brother’s requests. However, when Constantine continued to press his demands for territory in Africa, Constans struck. He arranged an ambush and had Constantine assassinated. The territories of Constantine II were then absorbed into the dominions of Constans.

Still a young man (probably about 17 when he disposed of his older brother) and now possessed of the entire West, Constans proved an energetic defender of the frontiers and a good ruler—at least at first. By AD 350, however, his disgusting personal vices and tendency to shower favors upon his barbarian guardsmen who committed crimes without check, blackened his name across much of his domain. The early 6th century historian, Zosimus, explains what happened next and how Magnentius rose to contend for the imperial power:

Constans, having thus removed his brother, exercised every species of cruelty toward his subjects, exceeding the most intolerable tyranny. He purchased some well favored Barbarians, and had others with him as hostages, to whom he gave liberty to harrass his subjects as they pleased, in order to gratify his vicious disposition. In this manner he reduced all the nations that were subject to him to extreme misery.

This gave uneasiness to the court guards, who perceiving that he was much addicted to hunting, placed themselves under the conduct of Marcellinus prefect of the treasury, and Magnentius who commanded the Joviani and Herculiani (two legions so termed), and formed a plot against him in the following manner.

Marcellinus reported that he meant to keep the birth-day of his sons, and invited many of the superior officers to a feast. Amongst the rest Magnentius rose from table and left the room. He presently returned, and as it were in a drama stood before them clothed in an imperial robe. Upon this all the guests saluted him with the title of king, and the inhabitants of Augustodunum, where it was done, concurred in the same sentiment. This transaction being rumored abroad, the country people flocked into the city, while at the same time, a party of Illyrian cavalry who came to supply the Celtic legions, joined themselves with those that were concerned in the enterprize. When the officers of the army were met together, and heard the leaders of the conspiracy proclaim their new emperor, they scarcely knew the meaning of it. They all, however, joined in the acclamation, and saluted Magnentius with the appellation of Augustus.

When this became known to Constans, he endeavored to escape to a small town called Helena, which lies near the Pyrenean mountains. He was taken by Gaison, who was sent with some other select persons for that purpose, and being destitute of all aid, was killed. [Zosimus, New History, Book II]

Magnentius soon found other western usurpers undercutting his authority. Vetranio was proclaimed emperor in Illyricum at the request of Constantia, daughter of Constantine the Great and sister of the slain Constans. At Rome, Nepotianus, the nephew of Constans and his brothers, raised a rebellion with a large band of gladiators. Magnentius soon quashed Nepotianus and opened negotiations with Vetranio while Constantius II was bogged down in a dangerous war with Persia on the eastern frontier.

This situation endured for a year and it seemed likely that Magnentius and Vetranio would be recognized as co-Augusti along with Constantius II. The son of Constantine, however, had other ideas and made a bold move, perhaps prompted by a miraculous vision of a cross in Jerusalem. Zosimus continues:

Meantime Constantius advanced from the east against Magnentius, but deemed it best first to win over Vetranio to his interest, as it was difficult to oppose two rebels at once. On the other hand, Magnentius used great endeavors to make Vetranio his friend, and thus to put an end to the war against Constantius. Both therefore sent agents to Vetranio, who chose to adopt the friendship of Constantius rather than that of Magnentius. The ambassadors of Magnentius returned without effecting their purpose.

Constantius desired that both armies might join, to undertake the war against Magnentius. To which proposal Vetranio readily assented, and they seated themselves on a throne provided for the occasion. Constantius, speaking first according to his dignity, endeavored to remind the soldiers of his father's munificence, and of the oaths they had taken to he true to his children. He then told them, that they ought not to suffer Magnentius to go unpunished, who had murdered the son of Constantine, with whom they had fought many battles, and had been generously remunerated. When the soldiers heard this, having been previously corrupted by valuable presents, they cried out, that they would have no mock emperors, and immediately began to strip the purple from Vetranio, and pulled him from the throne. [Zosimus, New History, Book II]

Reverse side of the above coin.
VOT V, MULT X on the shield refers
to vows taken by Magnentius to ensure
a five and ten year reign.

Constantius II showed mercy to the more aged Vetranio and allowed him to retire to an estate in Bithynia to live out the rest of his life.

This action set the stage for the great battle of Mursa Major. Constantius II marched his army into Pannonia and sought to bring Magnentius to battle. The cagey usurper withdrew, preferring to attempt to ambush the superior forces of Constantius. At least one of these ambushes achieved the desired result and weakened a part of Constantius’s army. This success convinced Magnentius to muster all his troops and attempt a decisive battle.

Alarmed, Constantius sent one of his bravest and most trusted officers, a man named Philip, into Magnentius's camp, ostensibly to treat for peace. In reality, Philip’s purpose was to determine the state of Magnentius’s army and to undermine their morale. This Philip did. As luck would have it, when Philip arrived at Magnentius’s camp, the whole army was drawn up in ranks. Philip immediately addressed himself to them in the following terms, according to the account of Zosimus:

[Philip began by telling the soldiers] that it did not become them, who were Roman subjects, to make war on Romans, especially as the emperor was the son of Constantine, with whom they had erected many trophies over the Barbarians. That Magnentius, moreover, ought to remember Constantine, and the kindness he had shewn to him and to his parents. That it was Constantine who had protected him when in imminent danger, and exalted him to the highest dignities. Having made these observations, he requested Magnentius to depart from Italy, and to be content with the government of the nations beyond the Alps. [Zosimus, New History, Book II]

This speech nearly caused the army of Magnentius to mutiny. In alarm, Magnentius addressed them, saying that he desired peace with Constantius and would confer with his officers to determine how best to bring this about. The next day, he addressed his army as follows:

He reminded them of the injuries they received from Constans when furious and intoxicated. That the soldiers could not sustain the enormities with which he oppressed the state contrary to all law and justice, but had inclined to what was most for the public advantage, and that after they had freed the cities from so savage a monster, they had compelled him to become their emperor. [Zosimus, New History, Book II]

Almost before Magnentius had concluded his speech, his army rose up and repented of their earlier near-rebellion. They declared themselves ready to fight immediately. With the hopes for a peaceful resolution waning, the two forces attempted to position themselves advantageously, retreating, feinting and advancing to choose a field of battle that best suited their particular strengths. From the account in Zosimus, it seems that Magnentius attempted to position his force between Constantius II and his base of operations in the east, taking cities along Constantinus's lines of communication. He was stymied, however, at the city of Mursa in Pannonia which held out against him. It was there that Constantius caught up with Magnentius and a full-scale battle ensued. Zosimus describes the unique lead-up to the conflict:

When he therefore heard that Constantius was near Mursa, [Magnentius] invented another stratagem to this effect. There was before the city a stadium or place of exercise, formerly used by those that fought for prizes, which was covered over with wood. In this he concealed four companies of Celtae, with orders when Constantius should come up, and they were ready to engage before the city, to attack the enemy by surprise, and to surround them and kill every man. This being discovered by those [friendly to Constantius] that were on the walls, Constantius immediately sent thither Scolidoas and Manadus, two of his officers. They first selected the choicest of their men, both heavy armed and archers, and taking them along with themselves, fastened up all the doors of the stadium. Having then possessed themselves of the upper steps loading into the Stadium, and inclosed the soldiers that were within on all sides, they threw darts at them. And observing some of them with their shields placed over their heads attempting to force open the doors, they fell upon them and did not cease throwing darts or cutting at them with their swords until they had killed them all. This project of Magnentius being thus frustrated, the armies met and engaged in the plain before Mursa, where such a battle was fought as had not occurred before in the course of this war, and great numbers fell on both sides...

At the beginning of the battle, it appears that some of Magnentius's men, led by a Frankish tribune named Silvanus, defected to the side of Constantius [Ammianus Marcellinus, Roman Antiquities, Book XV]. While this loss was injurious to his cause, Magnentius and his supporters fought on stoutly: 

...[The army] of Magnentius became more furious, nor would they cease fighting though night came on, but even their officers continued performing what belonged to common soldiers, and encouraging their men to oppose the enemy with vigor. On the other side likewise, the officers of Constantius called to mind the ancient bravery and renown of the Romans. Thus the battle continued until it was completely dark, nor did even darkness cause them to relax ; but they wounded each other with spears, swords or whatever was in their reach, so that neither night nor any other obstacle which usually causes some respite in war, could put an end to the slaughter, as if they thought it the greatest felicity that could happen to them to perish beside each other. Amongst the officers, that shewed great bravery in this battle and fell in it were Arcadius, commander of the legion called Abulci, and Menelaus, who commanded the Armenian horse archers. What is said of Menelaus is worthy of being related. He could take three darts at once, and with one shot hit three men, by which manner of shooting he killed a great, number of the enemy, and was himself almost the cause of their flight. He was killed by Romulus, who was the first in command in the army of Magnentius, and Romulus himself fell at the same time. He was wounded by a dart which Menelaus had thrown at him, yet continued fighting after he had received the wound, until he had killed the person who had given it to him.

Constantius now gaining the victory, by the army of Magnentius taking to flight, a terrible slaughter ensued. Magnentius, therefore being deprived of all hope, and apprehensive lest the remnant of his army should deliver him to Constantius, deemed it best to retire from Pannonia, and to enter Italy... [Zosimus, New History, Book II]

Magnentius mananged to survive another year before being cornered and slain at the battle of Mons Seleucus in southern Gaul in AD 353.

It is likely that Constantius felt that the Battle of Mursa Major, though a victory, was a horrible waste. Clearly, from the description above, many of the bravest imperial soldiers fell fighting each other, with some modern estimates going as high as 54,000 slain between both sides. Zosimus, writing with the advantage of historical hindsight, puts the thought in Constantius's mind that: "as this was a civil war, victory itself would be scarcely an advantage to him, now the Romans being so much weakened, as to be totally unable to resist the barbarians who attacked them on every side." 

After Magnentius's final defeat, Constantius II would go on to rule the Empire as the sole Augustus until AD 361 when his nephew, Julian the Apostate, rose against him. As an interesting side note, the wife of Magnentius—a woman named Justina—would go on to become the wife of the emperor Valentinian I, mother of Valentinian II, and become better known to history as the Arian persecutor of Saint Ambrose of Milan.