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Eastern Emperor Constantius II waged a costly war of revenge against his brother’s killer, Magnentius, seriously weakening the Roman Empire on the eve of the Goth invasions.

Of the many civil wars that periodically convulsed the late Roman Empire during the fourth century, the longest and most devastating began at a boy’s birthday party in the provincial Gallic town of Augustodunum (modern Autun in central France) on a winter night early in the year 350. On the evening of January 18, the members of the civil and military high command of the Western Roman Empire made their way to a hall reserved by Marcellinus, the emperor’s finance minister, who was holding a grand banquet in honor of his son’s birthday. For most of those attending, the occasion promised to be a convivial and welcome respite from the cold days and long nights characteristic of the season. They little suspected the larger purpose that lay behind the celebration.

The Western Roman emperor, Flavius Julius Constans I, did not attend, but his absence would have occasioned little surprise among most of the guests. Twenty-seven years old and never married, Constans had two passions: hunting and disporting himself with the handsome young barbarian slaves who made up a prominent part of his personal household. On the night Marcellinus scheduled his banquet, Constans and several of his favorites were away hunting in a nearby forest. The suspected nature of their diversions doubtless prompted some crude and ribald speculation from the high-ranking military officers and civil officials among Marcellinus’ guests.

Marcellinus provided an ample repast and plenty of wine, and he contrived to prolong the festivities until close to midnight. By the time the commander of the Joviani and Herculiani legions, Flavius Magnus Magnentius, rose from his table and excused himself briefly, many of the guests had reached a state of dulled inhibitions and easy suggestibility that left them perfectly primed for what was about to happen.

It appeared that Magnentius had merely excused himself to answer a routine call of nature. However, a few moments later the doors to the banqueting hall were suddenly flung open, stunning the increasingly rowdy guests into silence. For Magnentius now stepped forth as though on a stage, surrounded by soldiers and clad in purple robes like those of an emperor of Rome. His confederates among the guests loudly saluted him with the imperial title, while those not included in the plot momentarily vacillated, until conviction, ambition, intimidation, or intoxication prompted them to join in the acclamations.

Once they secured the support of those attending the banquet, the conspirators seized the palace and the imperial treasury; confirmed the loyalty of the troops camped outside the town by distributing cash, and dispatched a barbarian officer named Gaiso with a troop of cavalry to hunt down Constans in the forest. Someone warned Constans in time, but almost all of his companions deserted him, and he fled toward Spain, hoping to take a ship either to his remaining domains in Italy or Africa or to the more distant lands ruled by his brother Constantius II in the East.

Gaiso pursued him in a chase that lasted just over a week. On January 27, Gaiso and his men cornered their quarry in the small town of Castrum Helenae (modern Elne), near the Pyrenees in southwestern Gaul. Facing death, Constans, a devout Christian, sought refuge in a pagan temple. Gaiso and his men brushed aside any claim of sanctuary. They dragged their former master out and slaughtered him without compunction.

The deposition and murder of Constans started a fratricidal struggle between the Western and Eastern halves of the Roman Empire that lasted forty-three months. Before it ended, this civil war had consumed the equivalent of two good-sized field armies at a time when Rome faced growing threats from its barbarian neighbors and a renascent Persian Empire, and it left large tracts of the Rhineland and Gaul devastated. In the longer perspective of history, the usurpation of Magnentius marked the point at which the fortunes of the Western Roman Empire started a downward trend that would end half a century later with the virtual collapse of its defenses.

When Constans died, virtually alone and friendless, slaughtered in an obscure town in the Pyrenean foothills, he had ruled the largest part of the Roman world for nearly ten years. Constans assumed the imperial title along with his two older brothers, Constantine II and Constantius II, upon the death of their father, Constantine the Great, in May 337. In their original division of the empire, Constantine II ruled the West (Spain, Gaul, and Britain); Constans held the central provinces (Italy, Africa, Illyria, Pannonia, and Greece); while Constantius II ruled the eastern regions: Thrace, Anatolia, Syria, Palestine, and Egypt.

Less than three years after their father’s death, however, the overly ambitious elder brother, Constantine II, launched a reckless attack upon Constans’ dominions that ended in his own defeat and death in battle near Aquileia in northeastern Italy. Constans absorbed all his late brother’s possessions into his own realm and thus, at the age of only sixteen or seventeen, became the master of almost two-thirds of the Roman Empire.

Despite his youth, Constans began his reign well, campaigning vigorously and successfully against the empire’s barbarian enemies. However, by the end of the decade of the 340s, the young emperor had grown imperious, self-indulgent, and corrupt, selling offices to avaricious aspirants and awarding other posts to unqualified young favorites whose chief recommendation was that they had shared his bed. Constans’ misrule eventually produced a conspiracy against him that included many of the highest-ranking civil officials and military officers in his administration.

The central figure in the plot was not Magnentius himself but rather Marcellinus, one of the Western Empire’s principal financial officials. Because Marcellinus was responsible for acquiring, managing, and selling state properties and making cash payments from the imperial treasury, he was acutely aware of the favoritism and waste that now characterized Constans’ rule.

The conspirators presumably chose Magnentius to replace Constans because he had a strong following among the soldiers. Magnentius was born around 303 to a British father and a Frankish mother in Ambiana (Amiens). His father was a craftsman; his mother may have been a slave.

Magnentius lived among a Gallic tribe during his youth, but acquired a Latin education before entering the army under Constantine the Great. In January 350, he commanded two elite legions that served as the Western emperor’s personal guard. His subsequent record in the field against Constantius demonstrates that he was audacious, aggressive, and a hard fighter.

Once they had hunted down and killed Constans, the conspirators had a breathing space of six to eight weeks before the news could reach Constans’ surviving brother, Constantius II, at his Eastern capital, Antioch in Syria. If the conspirators could put themselves in a sufficiently strong position, it was possible that Constantius—who was already embroiled in a long-running and difficult war with the Persian Empire—would accept a negotiated resolution that left Magnentius in control in the West.

Magnentius accordingly mustered the Western Roman field army, crossed the Cottian Alps in midwinter, and advanced across the fertile plains of northern Italy. In early March he reached Aquileia, near the head of the Adriatic, and this city became his principal forward base for the next two years. Other troops loyal to him marched south into the Italian Peninsula and occupied Rome by the end of February.

As news of the coup spread eastward across the empire in the winter of 350, it became apparent that the respective fates of Magnentius and Constantius might ultimately turn on the attitude of the legions posted along the middle Danube in Pannonia. Vetranio, an elderly, rough-hewn general who possessed a solid military record, an unpretentious manner, and the admiration of his troops, commanded the Pannonian legions. A simple soldier who had risen through the ranks, Vetranio remained illiterate even after he succeeded to one of the empire’s most important military posts.

In early March 350, Vetranio’s restless troops declared him their emperor. This move may simply have reflected the Pannonian soldiers’ unwillingness to accept the leadership of a general less familiar to them, or perhaps Vetranio and his men believed that their move would avert a clash between Magnentius and Constantius that would otherwise cost tens of thousands of Roman soldiers their lives. Whatever the reasons for it, Vetranio’s acceptance of the imperial purple proved an enormous boon to Constantius, who was tied down in Syria by an impending Persian invasion. As long as Vetranio and his legions maintained their neutral stance, the army of Magnentius could not continue its march eastward down the Danube Valley to Constantinople.

Constantius spent the spring and summer of 350 at Edessa, anxiously awaiting the outcome of a massive Persian attack upon the Roman frontier fortress of Nisibis in upper Mesopotamia. Following an epic struggle that lasted several months, the badly mauled Persian army abandoned the siege and limped back across the Tigris River. Constantius first rebuilt Nisibis’ shattered walls, then gathered the bulk of his army at Antioch in the autumn of 350 and set off across the Tarsus Mountains and the treeless plains of the high Anatolian plateau for Constantinople.

In November 350, Constantius and his army crossed from Asia into Thrace, where he invited Vetranio to join him for a summit meeting at Serdica (modern Sofia). Vetranio agreed, and on Christmas Day 350, the two armies assembled on a plain outside Serdica, surrounding a raised tribunal, to be addressed by each of their leaders.

As the senior emperor in legitimacy and tenure, Constantius spoke first. Although he was an indifferent orator, on this single occasion Constantius appeared to surpass himself. He reminded the troops of his father’s bravery, military successes, and liberality to his soldiers, recounted the treacherous betrayal and murder of his brother Constans, asserted his own rights both to avenge his brother’s death and to inherit his dominions, and regretfully chided Vetranio’s men for their lack of fidelity.

From the ranks of the officers assembled at the foot of the tribunal first came sporadic shouts of agreement. The affirmative clamor slowly spread to the rank and file, gradually swelling in volume until the plain resounded with acclamations for Constantius and denunciations of the usurpers. Vetranio, appearing shaken and humbled, prostrated himself at his former master’s feet. Constantius responded with apparent magnanimity, lifting up the old veteran and saluting him as “Father,” and then turned with him to accept the cheers of their now-combined armies. Constantius later awarded Vetranio an estate near Prusa in Asia Minor, where he lived out the final six years of his life in comfortable retirement.

On its face, it was a dramatic episode. But agents of Constantius had spread gold among the officers in Vetranio’s army in advance of the Christmas Day assembly, and the shouts of acclamation that ultimately swayed the mass of the troops were choreographed in advance. The real question is whether Constantius had earlier flattered Vetranio with suggestions that he would recognize him as co-emperor, after which he secretly undermined and outmaneuvered the old general, or whether Vetranio always secretly maintained his allegiance to Constantius and only accepted the purple to forestall the possibility that his troops might otherwise declare their support for Magnentius.

After Vetranio’s army united with the troops of Constantius, the combined force marched up the great Balkans highway before going into winter quarters at Sirmium (Mitrovica) in the middle Danube valley. The Magnentian army wintered at Aquileia.

With Vetranio and his army no longer separating the two antagonists, a climactic showdown between Constantius and Magnentius seemed certain in 351. Magnentius drew the first blood. In the spring of 351, he seized one of the passes through the Julian Alps by treachery, then lured a detachment of Constantius’ army into an imprudent advance by sending a fake dispatch, purportedly coming from a friendly commander. As the loyalist forces proceeded without armor or adequate advance or flank guards, Magnentius ambushed and slaughtered them in a subalpine valley known as the Defiles of Atrans.

Magnentius then invaded Pannonia and commenced a campaign of maneuver down the river valleys of the Drave and Save, which flow southeast to their confluence with the Danube. His army was markedly smaller than the Constantian forces, particularly in cavalry. Magnentius sought to make up for what he lacked in numbers with maneuver, surprise, and aggressive self-confidence.

Marching first through Poetovio in the upper valley of the Drave, he suddenly knifed across the high ground dividing the two watersheds and appeared before the town of Siscia (Sisak) on the Save, where there was an imperial mint. When his attack on Siscia resulted in a bloody repulse, Magnentius moved his army back across the high ground between the rivers and resumed his advance down the valley of the Drave toward the Danube.

Constantius shifted his own army eastward and dug in at Cibalis (Vinkovci). Magnentius abruptly doubled back, reappeared before Siscia, and successfully stormed the town late in August. Magnentius then once again nimbly shifted his army to the southeast and moved against Mursa (Osijek), a town with a legionary camp and naval base on the Drave a dozen miles upstream from its juncture with the Danube.

When he arrived outside Mursa, Magnentius found the town alert, adequately garrisoned, and well-fortified—with strong walls and great iron-covered wooden gates. Since he lacked the heavy artillery necessary to mount a siege, Magnentius gambled on an immediate attack. His men tried to set Mursa’s wellprotected gates afire, but the defenders frustrated their efforts by pouring water from atop the gate tower, and the assault failed. Constantius meanwhile broke camp at Cibalis and marched to nearby Mursa’s relief.

Magnentius drew back his forces and prepared to face Constantius’ approaching army. As he did so, he noticed a long-abandoned stadium not far from Mursa that also stood reasonably close to the main road running through the Danube Valley. Once used for military drills and athletic competitions, it was now overgrown with saplings, vines, and bushes. Magnentius posted four companies of Celts in this picturesque ruin, ordering them to ambush their enemies once the army of Constantius approached the town.

Unfortunately for Magnentius, some of those inside Mursa observed this move and warned Constantius of what was afoot. The Eastern emperor dispatched a picked force of legionaries and archers to surround the ruined stadium. After foot soldiers sealed off its gates, the archers climbed up and swarmed over the upper tiers of ruined seats. Suddenly they began to shoot down on the surprised Magnentian soldiers massed inside. Magnentius’ troops put their shields over their heads in testudo (tortoise) formation and tried to force their way out through one of the gates. Most were slaughtered.

That their stratagem had backfired so disastrously must have damaged the morale of Magnentius’ army. And soon the Western army had more bad news: The respected Frankish General Silvanus deserted to Constantius with a regiment of elite heavy cavalry, further weakening Magnentius’ already inferior mounted arm.

The long-awaited confrontation between the two armies came on September 28, 351. Magnentius’ army mustered no more than thirty-six thousand men against a reported eighty thousand under Constantius. This vast numerical disparity left Magnentius with little alternative but to stand on the defensive as the two armies faced off on the plain beside the Drave.

Magnentius’ army took its position with the broad river on its left, thus ensuring that this flank, at least, was well protected. He posted the Western army’s remaining cavalry, the traditional lightly armored Roman horsemen, to cover its right flank.

Constantius’ army formed up with its heavy infantry massed in the center, and its slingers, foot archers, and other light skirmishers who operated without shields or armor to their rear. The armored cataphracts, or heavy cavalry with their long lances, occupied the flanks on either side, with the lighter Eastern horse archers deployed behind them, awaiting the opportunity to dash through any gaps the cataphracts could punch in the enemy’s ranks. Since the Constantian army was significantly larger, the left end of its line extended some distance beyond the right flank of its Western enemies.

The Eastern army’s cataphracts were a new element in intra-Roman combat. Inspired by examples drawn from the Persian Empire and the Sarmatian barbarians, this specialized cavalry arm comprised one of Constantius’ most significant assets. Sheathed in carefully fitted mail and plate armor, the cataphracti were the earliest forerunners of the armored knights of the high Middle Ages. Contemporary observers marveled at the elaborate equipment and intimidating appearance of these “Ironclads,” as this passage from an account of the battle of Mursa indicates. Julian, Constantius’ cousin and eventual successor, wrote:

[Constantius’] cavalry was almost unlimited in numbers and they sat their horses like statues, while their limbs were fitted with [scale] armor that followed closely the outline of the human form. It covers the arms from wrist to elbow and thence to the shoulder, while a coat of mail protects the shoulders, back and breast. The head and face are covered by a metal mask which makes its wearer look like a glittering statue, for not even the thighs and legs and the very ends of the feet lack this armor. It is attached to the cuirass by fine chain-armor like a web, so that no part of the body is visible and uncovered, for this woven covering protects the hands as well, and is so flexible that the wearers can even bend their fingers.

The opposing armies stood at arms for much of the day, perhaps because the Eastern force had not yet fully arrived upon the field that morning. Late that afternoon the action began on the left, as the Eastern cavalry advanced in an oblique attack against the Magnentians’ right flank. The Western army initially recoiled from the impact of this charge, then recovered and stood its ground. Once the battle was joined, it continued for the remainder of the day and well into the night. Both sides fought with ferocity and determination, leaving the outcome long in doubt. Even Julian’s account acknowledged “the extraordinary daring of the usurper’s troops in the face of dangers and their great eagerness to come to close quarters.”

Given the serious handicaps under which the Western army labored—heavily outnumbered, its morale doubtless affected by recent defeats and defections, and its cavalry greatly inferior in numbers, in protection, and in weaponry—it is remarkable that the army of Magnentius sustained the struggle for as long and as well as it did. Nevertheless, the sheer weight of numbers eventually told. As the Eastern cataphracts with their long lances gradually broke apart the solid line of the Gallic legions, the lighter Asiatic cavalry moved in to exploit the openings created, and the Western army’s right flank and center were slowly pressed back toward the Drave.

When an ancient army’s front line fragmented, the end usually came quickly. However, even Constantius’ cousin Julian could not withhold his admiration for the way Magnentius’ men fought during the battle’s final stages. When the Western army’s line at length began to break apart, he wrote:

What happened then was beyond all expectation; for the enemy refused altogether to yield to those who were defeating them, while our men did their utmost to achieve a signal victory, and so there arose the wildest confusion, loud shouts mingled with the din of weapons, as swords were shattered against helmets and shields against spears. It was a hand to hand fight, in which [the Magnentians] discarded their shields and attacked with swords only, while, indifferent to their fate, and devoting the utmost ardor to inflicting severe loss upon the foe, they were ready to meet even death if only they could make our victory seem dearly bought. It was not only the infantry who behaved thus to their pursuers, but even the cavalry, whose spears were broken and were now entirely useless….[T]hey dismounted and transformed themselves into infantry. So for some time they held their own against the greatest odds.

However, the incessant rain of arrows from the Eastern army’s horse archers and the repeated charges by its well-protected cataphracti ultimately did their work. Sometime well into the night, the Western army’s resistance crumbled at last, and its surviving soldiers sought safety in flight. The horsemen of Constantius pursued the right wing’s fugitives as far as Magnentius’ camp, which like all Roman military bivouacs was stoutly fortified. But Constantius successfully stormed the lightly held camp and captured the Western army’s baggage, slaves, pack animals, and wagons. At that point, any organized retreat became impossible.

As the Western army’s remaining lines dissolved and the cavalry of Constantius moved to cut off the retreat route up the Drave, Magnentius and his high command decided that he, at least, must escape. He removed his imperial regalia, draped it over the saddle of his charger, and let the horse run free, so those who saw it would assume he was already dead or seriously wounded. He then fled up the river road on another horse and narrowly escaped capture before the trap closed.

Behind him, the surviving members of his high command gathered a corps of diehard troops for a last stand. Julian reports that Magnentius “left there his cavalry commander and his numerous centurions and tribunes, who continued to fight bravely, and in command of all these the real author of that monstrous and unholy drama”: Marcellinus, the architect of Magnentius’ usurpation.

During the last phase of the fighting, the commander of Constantius’ Armenian horse archers—an officer reputed to be so skilled with the bow that he could let three shafts fly at once—led a fierce attack against the Western army’s remaining center of resistance. He and his archers killed a great number of the enemy, “and was himself almost the cause of their flight.” One of his arrows mortally wounded the master of horse for the Western Roman army, but the cavalry leader continued fighting at what was obviously extremely close quarters until he killed the leader of the horse archers as well. The commander of the Eastern Roman legion known as the Abulci was also slain in this climactic phase of the battle.

It is practically certain that Marcellinus likewise died during the last stand of the Magnentian rear guard. But despite determined efforts after the battle to locate his body or ascertain the circumstances of his death, nothing conclusive could be learned. The frustration of Constantius and his officers is evident in Julian’s comments a few years later: “For the fact is that we do not know what [Marcellinus] did or suffered before he vanished out of sight, out of our ken…. For till the battle actually began, and while the troops were forming the phalanx, he was full of confidence and went to and fro in the center of their line. But when the battle was ended as was fitting, he vanished completely, taken from our sight by I know not what agency….”

He most likely died at or in the river, where the remains of the left and center of Magnentius’ army were “crowded together like a herd of oxen or brute beasts.” Julian wrote afterward that it was there that “the greatest slaughter took place, and the river was choked with the bodies of men and horses, indiscriminately.” Probably the current swept away Marcellinus’ body, along with hundreds of others.

The following morning found the two rival emperors in very different circumstances. Constantius, who had passed the battle praying in a church with the Bishop of Mursa, surveyed the plain and the river from an elevated spot, where he wept with grief and horror at the sight of tens of thousands of corpses and maimed men calling for assistance or surcease. He ordered that the dead of both armies be buried together and that the wounded be given equal medical care.

As for Magnentius, he rallied what he could from the wreck of his army and sought safety beyond the formidable ramparts of the Julian Alps, hotly pursued by the light cavalry of Constantius. Hoards of his coins found around Emona (modern Ljubljana) provide evidence of the hasty flight of his rearechelon officers or administrators. Constantius made no attempt to force the passes before the end of the year, and Magnentius settled in for the winter at his previous base, Aquileia.

The scale of the carnage at Mursa horrified contemporaries, since it was exceptional even by the sanguinary standards of the fourth century, with its repeated civil wars. Writing just a few years afterward, the historian Eutropius mourned that “Vast forces of the Roman empire were cut off in that struggle, sufficient for any foreign wars, and for procuring many triumphs, and a lasting peace.” Byzantine historian Ioannis Zonaras reported that in the single day of slaughter beside the Drave, Constantius lost thirty thousand men out of eighty thousand, while Magnentius suffered twenty-four thousand casualties out of his far smaller army of thirty-six thousand.

The numbers cited for the overall size of the respective armies are strikingly similar to those at Antietam, where the Army of the Potomac mustered some seventy-five thousand men and the Army of Northern Virginia thirty-eight thousand. But the twenty-three thousand combined casualties suffered by George B. McClellan’s and Robert E. Lee’s armies on the bloodiest day in American military history are dwarfed by the reported butcher’s bill of fifty-four thousand amassed by the Roman armies of Constantius and Magnentius on another September day some fifteen hundred years earlier.

The decision Magnentius made to take the war to Constantius in the Danube Valley, notwithstanding his significantly smaller force, had been a long chance— but his gamble was not unreasonable. Constantius had had an undistinguished military record in the East against the Persians, and the loyalty of Vetranio’s former troops was open to question. Magnentius may have hoped that if he could secure one or more quick victories through maneuver and surprise, the Constantian army would suffer wholesale defections, reducing the odds or even shifting them in his favor before the decisive encounter. But having gambled and lost, the odds facing Magnentius at the start of the third year of his usurpation were now very long indeed.

The campaign of 352 got off to a slow start because an incursion of Sarmatians across the Danube into his rear distracted Constantius. In August, however, Constantius launched a multipronged offensive, by both land and sea. His army first used a little-known path—and possibly treachery—to surround and bloodlessly capture a detachment of Western troops who guarded a key pass through the Julian Alps. Next, one of Constantius’ naval squadrons entered the Po River at its mouth on the upper Adriatic and moved upstream, breaking Magnentius’ line of communications with his garrisons farther south on the Italian Peninsula. Constantius also dispatched emissaries by sea to the Italian cities, many of which responded by unfurling his banners from their walls. The defections of these Italian garrisons further augmented the overwhelming numerical superiority Constantius now enjoyed.

Rather than accepting a siege at Aquileia, Magnentius decided upon a fighting withdrawal across northern Italy back to his original base in Gaul, which had strong natural defenses on all sides. In the course of this retreat he turned upon his overconfident and incautious pursuers near Ticinum (Pavia) on the upper Po, where he won a satisfying but ultimately useless victory.

As the campaigning season ended in 352, Magnentius’ position was crumbling rapidly. He sent ambassadors to Constantius, offering to trade the peaceful surrender of his remaining provinces—Africa, Spain, Gaul, and England—for his life, but Constantius refused to treat with him. Many of his original confederates were dead, having perished on the field at Mursa. Julian’s claim that on his return to Gaul, Magnentius tortured dissidents by dragging them behind racing chariots suggests that there were plots against him in the winter and spring of 352-53. The strikingly crude, debased coinage produced by some of his mints during the last year of his rule give evidence to his regime’s desperate financial straits.

In the spring of 353, Constantius launched a meticulously planned four-pronged assault to finish off Magnentius. One imperial fleet sailed from Egypt to Italy to take on troops, then crossed to North Africa and put an end to Magnentius’ rule in Carthage and Mauretania. Another loyalist fleet disembarked an army in Spain that moved northeast, marched over the Pyrenees, and invaded Gaul from the south.

Constantius also paid the barbarian tribes of Germany to attack Magnentius’ younger brother, Caesar Flavius Magnus Decentius, who was responsible for guarding the Western Empire’s northeastern frontier along the Rhine. Finally, Constantius sent emissaries to various tribal chieftains and prominent men within Gaul itself, urging them to revolt against Magnentius once the final campaign began.

Constantius’ varied efforts met with considerable success. A contemporary historian recorded that King Chnodomar of the Alemanni “defeated the Caesar Decentius in a battle fought on equal terms,…destroyed and sacked many rich towns, and for a long time ranged at will through Gaul with none to oppose him.” When Decentius fell back on Augusta Treverorum (Trier), he discovered that the townspeople had revolted in favor of Constantius and had closed the city’s gates against him. Decentius gathered what troops he could and made his way south to join his elder brother, but revolts by other Belgic and Gallic cities and garrisons forced him into circuitous detours that delayed his progress until it was too late.

When the coming of summer warmed the Alpine heights, Constantius launched his main attack. His principal army forced its way through the Cottian Alps in July, probably by the low passes of Mont-Genevre or the Col de Larche. With characteristic aggressiveness, Magnentius launched a final attack with his shrunken little army against the much larger force of Constantius at Mons Seleucus, near Vapincum (Gap) in the valley of the Isere. But Constantius’ troops easily brushed aside this last-ditch effort.

Magnentius then fell back up the Rhône River to Lugdunum (Lyon), still hoping to link up with his brother Decentius and the troops he was bringing down from the north. On August 10 or 11, however, Magnentius’ remaining soldiers mutinied and refused to follow his orders any longer. The increasingly desperate usurper mounted a platform and tried to harangue them back to obedience, but the effort proved hopeless: They shouted him down with loud and bitter acclamations for Constantius.

Magnentius retired to a house in Lugdunum with the members of his family and those retainers who still remained loyal. His troops surrounded the house, intending to offer him to Constantius in return for their own pardons. Realizing that the end was at hand, Magnentius— who seems by this point to have been virtually mad—decided to terminate his life in the manner of the ancient Gauls. He first slew his mother, then turned his sword against his relatives and remaining servants before killing himself. (One of his brothers survived, although he was seriously wounded. Constantius afterward pardoned him.)

A week later, word of Magnentius’ fate reached his brother Decentius at Agedincum (Sens), on the road between Lutetia (Paris) and Augustodunum. On August 18, with enemies closing in from all sides, Decentius hanged himself.

The usurpation of Magnentius was the first and most devastating of a succession of reverses and disasters that seriously weakened the Roman Empire on the eve of the great barbarian migrations that began just twenty years later. In 359-60 the Persian War resumed, and several important fortresses in Roman Mesopotamia unexpectedly fell to the Persians, with the loss of all their garrisons. In the summer of 363, the Roman armies suffered additional heavy losses in an unsuccessful invasion of Persia led by Julian, the successor to Constantius and the emperor known to history as “the Apostate” for his quixotic attempt to reestablish paganism as the Roman Empire’s state religion.

A few years later, in 367-68, the province of Britannia was temporarily overrun by coordinated barbarian attacks from land and sea. Roman Africa suffered severely from desert raiders, revolts, and domestic insurrections between 364 and 372.

Finally, in August 378, Visigoth and Ostrogoth tribesmen defeated the Eastern Roman army and killed the Emperor Valens at the Battle of Adrianople—a disaster that left two-thirds of the Eastern army’s men on the field dead and wounded. After this catastrophe, the empire never fully recovered, and no one could completely restore its territorial integrity. The Goths had started along the road that would lead them, after thirty-two more years of wandering and fighting, to the conquest of Rome itself.


JEFFERSON M. GRAY is a Baltimore attorney who previously has written for MHQ about the Acrocorinth in Greece.

Originally published in the Winter 2007 issue of Military History Quarterly. To subscribe, click here