The fate of Constans himself was delayed about ten years longer, and the revenge of his brother's death was reserved for the more ignoble hand of a domestic traitor. The pernicious tendency of the system introduced by Constantine was displayed in the feeble administration of his sons; who, by their vices and weakness, soon lost the esteem and affections of their people. The pride assumed by Constans, from the unmerited success of his arms, was rendered more contemptible by his want of abilities and application. His fond partiality towards some German captives, distinguished only by the charms of youth, was an object of scandal to the people; and Magnentius, an ambitious soldier, who was himself of Barbarian extraction, was encouraged by the public discontent to assert the honor of the Roman name. The chosen bands of Jovians and Herculians, who acknowledged Magnentius as their leader, maintained the most respectable and important station in the Imperial camp. The friendship of Marcellinus, count of the sacred largesses, supplied with a liberal hand the means of seduction. The soldiers were convinced by the most specious arguments, that the republic summoned them to break the bonds of hereditary servitude; and, by the choice of an active and vigilant prince, to reward the same virtues which had raised the ancestors of the degenerate Constans from a private condition to the throne of the world. As soon as the conspiracy was ripe for execution, Marcellinus, under the pretence of celebrating his son's birthday, gave a splendid entertainment to the illustrious and honorablepersons of the court of Gaul, which then resided in the city of Autun. The intemperance of the feast was artfully protracted till a very late hour of the night; and the unsuspecting guests were tempted to indulge themselves in a dangerous and guilty freedom of conversation. On a sudden the doors were thrown open, and Magnentius, who had retired for a few moments, returned into the apartment, invested with the diadem and purple. The conspirators instantly saluted him with the titles of Augustus and Emperor. The surprise, the terror, the intoxication, the ambitious hopes, and the mutual ignorance of the rest of the assembly, prompted them to join their voices to the general acclamation. The guards hastened to take the oath of fidelity; the gates of the town were shut; and before the dawn of day, Magnentius became master of the troops and treasure of the palace and city of Autun. By his secrecy and diligence he entertained some hopes of surprising the person of Constans, who was pursuing in the adjacent forest his favorite amusement of hunting, or perhaps some pleasures of a more private and criminal nature. The rapid progress of fame allowed him, however, an instant for flight, though the desertion of his soldiers and subjects deprived him of the power of resistance. Before he could reach a seaport in Spain, where he intended to embark, he was overtaken near Helena, at the foot of the Pyrenees, by a party of light cavalry, whose chief, regardless of the sanctity of a temple, executed his commission by the murder of the son of Constantine.
As soon as the death of Constans had decided this easy but important revolution, the example of the court of Autun was imitated by the provinces of the West. The authority of Magnentius was acknowledged through the whole extent of the two great præfectures of Gaul and Italy; and the usurper prepared, by every act of oppression, to collect a treasure, which might discharge the obligation of an immense donative, and supply the expenses of a civil war. The martial countries of Illyricum, from the Danube to the extremity of Greece, had long obeyed the government of Vetranio, an aged general, beloved for the simplicity of his manners, and who had acquired some reputation by his experience and services in war. Attached by habit, by duty, and by gratitude, to the house of Constantine, he immediately gave the strongest assurances to the only surviving son of his late master, that he would expose, with unshaken fidelity, his person and his troops, to inflict a just revenge on the traitors of Gaul. But the legions of Vetranio were seduced, rather than provoked, by the example of rebellion; their leader soon betrayed a want of firmness, or a want of sincerity; and his ambition derived a specious pretence from the approbation of the princess Constantina. That cruel and aspiring woman, who had obtained from the great Constantine, her father, the rank of Augusta, placed the diadem with her own hands on the head of the Illyrian general; and seemed to expect from his victory the accomplishment of those unbounded hopes, of which she had been disappointed by the death of her husband Hannibalianus. Perhaps it was without the consent of Constantina, that the new emperor formed a necessary, though dishonorable, alliance with the usurper of the West, whose purple was so recently stained with her brother's blood.
The intelligence of these important events, which so deeply affected the honor and safety of the Imperial house, recalled the arms of Constantius from the inglorious prosecution of the Persian war. He recommended the care of the East to his lieutenants, and afterwards to his cousin Gallus, whom he raised from a prison to a throne; and marched towards Europe, with a mind agitated by the conflict of hope and fear, of grief and indignation. On his arrival at Heraclea in Thrace, the emperor gave audience to the ambassadors of Magnentius and Vetranio. The first author of the conspiracy Marcellinus, who in some measure had bestowed the purple on his new master, boldly accepted this dangerous commission; and his three colleagues were selected from the illustrious personages of the state and army. These deputies were instructed to soothe the resentment, and to alarm the fears, of Constantius. They were empowered to offer him the friendship and alliance of the western princes, to cement their union by a double marriage; of Constantius with the daughter of Magnentius, and of Magnentius himself with the ambitious Constantina; and to acknowledge in the treaty the preeminence of rank, which might justly be claimed by the emperor of the East. Should pride and mistaken piety urge him to refuse these equitable conditions, the ambassadors were ordered to expatiate on the inevitable ruin which must attend his rashness, if he ventured to provoke the sovereigns of the West to exert their superior strength; and to employ against him that valor, those abilities, and those legions, to which the house of Constantine had been indebted for so many triumphs. Such propositions and such arguments appeared to deserve the most serious attention; the answer of Constantius was deferred till the next day; and as he had reflected on the importance of justifying a civil war in the opinion of the people, he thus addressed his council, who listened with real or affected credulity: "Last night," said he, "after I retired to rest, the shade of the great Constantine, embracing the corpse of my murdered brother, rose before my eyes; his well-known voice awakened me to revenge, forbade me to despair of the republic, and assured me of the success and immortal glory which would crown the justice of my arms." The authority of such a vision, or rather of the prince who alleged it, silenced every doubt, and excluded all negotiation. The ignominious terms of peace were rejected with disdain. One of the ambassadors of the tyrant was dismissed with the haughty answer of Constantius; his colleagues, as unworthy of the privileges of the law of nations, were put in irons; and the contending powers prepared to wage an implacable war.
Such was the conduct, and such perhaps was the duty, of the brother of Constans towards the perfidious usurper of Gaul. The situation and character of Vetranio admitted of milder measures; and the policy of the Eastern emperor was directed to disunite his antagonists, and to separate the forces of Illyricum from the cause of rebellion. It was an easy task to deceive the frankness and simplicity of Vetranio, who, fluctuating some time between the opposite views of honor and interest, displayed to the world the insincerity of his temper, and was insensibly engaged in the snares of an artful negotiation. Constantius acknowledged him as a legitimate and equal colleague in the empire, on condition that he would renounce his disgraceful alliance with Magnentius, and appoint a place of interview on the frontiers of their respective provinces; where they might pledge their friendship by mutual vows of fidelity, and regulate by common consent the future operations of the civil war. In consequence of this agreement, Vetranio advanced to the city of Sardica, at the head of twenty thousand horse, and of a more numerous body of infantry; a power so far superior to the forces of Constantius, that the Illyrian emperor appeared to command the life and fortunes of his rival, who, depending on the success of his private negotiations, had seduced the troops, and undermined the throne, of Vetranio. The chiefs, who had secretly embraced the party of Constantius, prepared in his favor a public spectacle, calculated to discover and inflame the passions of the multitude. The united armies were commanded to assemble in a large plain near the city. In the centre, according to the rules of ancient discipline, a military tribunal, or rather scaffold, was erected, from whence the emperors were accustomed, on solemn and important occasions, to harangue the troops. The well-ordered ranks of Romans and Barbarians, with drawn swords, or with erected spears, the squadrons of cavalry, and the cohorts of infantry, distinguished by the variety of their arms and ensigns, formed an immense circle round the tribunal; and the attentive silence which they preserved was sometimes interrupted by loud bursts of clamor or of applause. In the presence of this formidable assembly, the two emperors were called upon to explain the situation of public affairs: the precedency of rank was yielded to the royal birth of Constantius; and though he was indifferently skilled in the arts of rhetoric, he acquitted himself, under these difficult circumstances, with firmness, dexterity, and eloquence. The first part of his oration seemed to be pointed only against the tyrant of Gaul; but while he tragically lamented the cruel murder of Constans, he insinuated, that none, except a brother, could claim a right to the succession of his brother. He displayed, with some complacency, the glories of his Imperial race; and recalled to the memory of the troops the valor, the triumphs, the liberality of the great Constantine, to whose sons they had engaged their allegiance by an oath of fidelity, which the ingratitude of his most favored servants had tempted them to violate. The officers, who surrounded the tribunal, and were instructed to act their part in this extraordinary scene, confessed the irresistible power of reason and eloquence, by saluting the emperor Constantius as their lawful sovereign. The contagion of loyalty and repentance was communicated from rank to rank; till the plain of Sardica resounded with the universal acclamation of "Away with these upstart usurpers! Long life and victory to the son of Constantine! Under his banners alone we will fight and conquer." The shout of thousands, their menacing gestures, the fierce clashing of their arms, astonished and subdued the courage of Vetranio, who stood, amidst the defection of his followers, in anxious and silent suspense. Instead of embracing the last refuge of generous despair, he tamely submitted to his fate; and taking the diadem from his head, in the view of both armies fell prostrate at the feet of his conqueror. Constantius used his victory with prudence and moderation; and raising from the ground the aged suppliant, whom he affected to style by the endearing name of Father, he gave him his hand to descend from the throne. The city of Prusa was assigned for the exile or retirement of the abdicated monarch, who lived six years in the enjoyment of ease and affluence. He often expressed his grateful sense of the goodness of Constantius, and, with a very amiable simplicity, advised his benefactor to resign the sceptre of the world, and to seek for content (where alone it could be found) in the peaceful obscurity of a private condition.
The behavior of Constantius on this memorable occasion was celebrated with some appearance of justice; and his courtiers compared the studied orations which a Pericles or a Demosthenes addressed to the populace of Athens, with the victorious eloquence which had persuaded an armed multitude to desert and depose the object of their partial choice. The approaching contest with Magnentius was of a more serious and bloody kind. The tyrant advanced by rapid marches to encounter Constantius, at the head of a numerous army, composed of Gauls and Spaniards, of Franks and Saxons; of those provincials who supplied the strength of the legions, and of those barbarians who were dreaded as the most formidable enemies of the republic. The fertile plains of the Lower Pannonia, between the Drave, the Save, and the Danube, presented a spacious theatre; and the operations of the civil war were protracted during the summer months by the skill or timidity of the combatants. Constantius had declared his intention of deciding the quarrel in the fields of Cibalis, a name that would animate his troops by the remembrance of the victory, which, on the same auspicious ground, had been obtained by the arms of his father Constantine. Yet by the impregnable fortifications with which the emperor encompassed his camp, he appeared to decline, rather than to invite, a general engagement. It was the object of Magnentius to tempt or to compel his adversary to relinquish this advantageous position; and he employed, with that view, the various marches, evolutions, and stratagems, which the knowledge of the art of war could suggest to an experienced officer. He carried by assault the important town of Siscia; made an attack on the city of Sirmium, which lay in the rear of the Imperial camp, attempted to force a passage over the Save into the eastern provinces of Illyricum; and cut in pieces a numerous detachment, which he had allured into the narrow passes of Adarne. During the greater part of the summer, the tyrant of Gaul showed himself master of the field. The troops of Constantius were harassed and dispirited; his reputation declined in the eye of the world; and his pride condescended to solicit a treaty of peace, which would have resigned to the assassin of Constans the sovereignty of the provinces beyond the Alps. These offers were enforced by the eloquence of Philip the Imperial ambassador; and the council as well as the army of Magnentius were disposed to accept them. But the haughty usurper, careless of the remonstrances of his friends, gave orders that Philip should be detained as a captive, or, at least, as a hostage; while he despatched an officer to reproach Constantius with the weakness of his reign, and to insult him by the promise of a pardon if he would instantly abdicate the purple. "That he should confide in the justice of his cause, and the protection of an avenging Deity," was the only answer which honor permitted the emperor to return. But he was so sensible of the difficulties of his situation, that he no longer dared to retaliate the indignity which had been offered to his representative. The negotiation of Philip was not, however, ineffectual, since he determined Sylvanus the Frank, a general of merit and reputation, to desert with a considerable body of cavalry, a few days before the battle of Mursa.
The city of Mursa, or Essek, celebrated in modern times for a bridge of boats, five miles in length, over the River Drave, and the adjacent morasses, has been always considered as a place of importance in the wars of Hungary. Magnentius, directing his march towards Mursa, set fire to the gates, and, by a sudden assault, had almost scaled the walls of the town. The vigilance of the garrison extinguished the flames; the approach of Constantius left him no time to continue the operations of the siege; and the emperor soon removed the only obstacle that could embarrass his motions, by forcing a body of troops which had taken post in an adjoining amphitheatre. The field of battle round Mursa was a naked and level plain: on this ground the army of Constantius formed, with the Drave on their right; while their left, either from the nature of their disposition, or from the superiority of their cavalry, extended far beyond the right flank of Magnentius. The troops on both sides remained under arms, in anxious expectation, during the greatest part of the morning; and the son of Constantine, after animating his soldiers by an eloquent speech, retired into a church at some distance from the field of battle, and committed to his generals the conduct of this decisive day. They deserved his confidence by the valor and military skill which they exerted. They wisely began the action upon the left; and advancing their whole wing of cavalry in an oblique line, they suddenly wheeled it on the right flank of the enemy, which was unprepared to resist the impetuosity of their charge. But the Romans of the West soon rallied, by the habits of discipline; and the Barbarians of Germany supported the renown of their national bravery. The engagement soon became general; was maintained with various and singular turns of fortune; and scarcely ended with the darkness of the night. The signal victory which Constantius obtained is attributed to the arms of his cavalry. His cuirassiers are described as so many massy statues of steel, glittering with their scaly armor, and breaking with their ponderous lances the firm array of the Gallic legions. As soon as the legions gave way, the lighter and more active squadrons of the second line rode sword in hand into the intervals, and completed the disorder. In the mean while, the huge bodies of the Germans were exposed almost naked to the dexterity of the Oriental archers; and whole troops of those Barbarians were urged by anguish and despair to precipitate themselves into the broad and rapid stream of the Drave. The number of the slain was computed at fifty-four thousand men, and the slaughter of the conquerors was more considerable than that of the vanquished; a circumstance which proves the obstinacy of the contest, and justifies the observation of an ancient writer, that the forces of the empire were consumed in the fatal battle of Mursa, by the loss of a veteran army, sufficient to defend the frontiers, or to add new triumphs to the glory of Rome. Notwithstanding the invectives of a servile orator, there is not the least reason to believe that the tyrant deserted his own standard in the beginning of the engagement. He seems to have displayed the virtues of a general and of a soldier till the day was irrecoverably lost, and his camp in the possession of the enemy. Magnentius then consulted his safety, and throwing away the Imperial ornaments, escaped with some difficulty from the pursuit of the light horse, who incessantly followed his rapid flight from the banks of the Drave to the foot of the Julian Alps.
The approach of winter supplied the indolence of Constantius with specious reasons for deferring the prosecution of the war till the ensuing spring. Magnentius had fixed his residence in the city of Aquileia, and showed a seeming resolution to dispute the passage of the mountains and morasses which fortified the confines of the Venetian province. The surprisal of a castle in the Alps by the secret march of the Imperialists, could scarcely have determined him to relinquish the possession of Italy, if the inclinations of the people had supported the cause of their tyrant. But the memory of the cruelties exercised by his ministers, after the unsuccessful revolt of Nepotian, had left a deep impression of horror and resentment on the minds of the Romans. That rash youth, the son of the princess Eutropia, and the nephew of Constantine, had seen with indignation the sceptre of the West usurped by a perfidious barbarian. Arming a desperate troop of slaves and gladiators, he overpowered the feeble guard of the domestic tranquillity of Rome, received the homage of the senate, and assuming the title of Augustus, precariously reigned during a tumult of twenty-eight days. The march of some regular forces put an end to his ambitious hopes: the rebellion was extinguished in the blood of Nepotian, of his mother Eutropia, and of his adherents; and the proscription was extended to all who had contracted a fatal alliance with the name and family of Constantine. But as soon as Constantius, after the battle of Mursa, became master of the sea-coast of Dalmatia, a band of noble exiles, who had ventured to equip a fleet in some harbor of the Adriatic, sought protection and revenge in his victorious camp. By their secret intelligence with their countrymen, Rome and the Italian cities were persuaded to display the banners of Constantius on their walls. The grateful veterans, enriched by the liberality of the father, signalized their gratitude and loyalty to the son. The cavalry, the legions, and the auxiliaries of Italy, renewed their oath of allegiance to Constantius; and the usurper, alarmed by the general desertion, was compelled, with the remains of his faithful troops, to retire beyond the Alps into the provinces of Gaul. The detachments, however, which were ordered either to press or to intercept the flight of Magnentius, conducted themselves with the usual imprudence of success; and allowed him, in the plains of Pavia, an opportunity of turning on his pursuers, and of gratifying his despair by the carnage of a useless victory.
The pride of Magnentius was reduced, by repeated misfortunes, to sue, and to sue in vain, for peace. He first despatched a senator, in whose abilities he confided, and afterwards several bishops, whose holy character might obtain a more favorable audience, with the offer of resigning the purple, and the promise of devoting the remainder of his life to the service of the emperor. But Constantius, though he granted fair terms of pardon and reconciliation to all who abandoned the standard of rebellion, avowed his inflexible resolution to inflict a just punishment on the crimes of an assassin, whom he prepared to overwhelm on every side by the effort of his victorious arms. An Imperial fleet acquired the easy possession of Africa and Spain, confirmed the wavering faith of the Moorish nations, and landed a considerable force, which passed the Pyrenees, and advanced towards Lyons, the last and fatal station of Magnentius. The temper of the tyrant, which was never inclined to clemency, was urged by distress to exercise every act of oppression which could extort an immediate supply from the cities of Gaul. Their patience was at length exhausted; and Treves, the seat of Prætorian government, gave the signal of revolt, by shutting her gates against Decentius, who had been raised by his brother to the rank either of Cæsar or of Augustus. From Treves, Decentius was obliged to retire to Sens, where he was soon surrounded by an army of Germans, whom the pernicious arts of Constantius had introduced into the civil dissensions of Rome. In the mean time, the Imperial troops forced the passages of the Cottian Alps, and in the bloody combat of Mount Seleucus irrevocably fixed the title of rebels on the party of Magnentius. He was unable to bring another army into the field; the fidelity of his guards was corrupted; and when he appeared in public to animate them by his exhortations, he was saluted with a unanimous shout of "Long live the emperor Constantius!" The tyrant, who perceived that they were preparing to deserve pardon and rewards by the sacrifice of the most obnoxious criminal, prevented their design by falling on his sword; a death more easy and more honorable than he could hope to obtain from the hands of an enemy, whose revenge would have been colored with the specious pretence of justice and fraternal piety. The example of suicide was imitated by Decentius, who strangled himself on the news of his brother's death. The author of the conspiracy, Marcellinus, had long since disappeared in the battle of Mursa, and the public tranquillity was confirmed by the execution of the surviving leaders of a guilty and unsuccessful faction. A severe inquisition was extended over all who, either from choice or from compulsion, had been involved in the cause of rebellion. Paul, surnamed Catena from his superior skill in the judicial exercise of tyranny, * was sent to explore the latent remains of the conspiracy in the remote province of Britain. The honest indignation expressed by Martin, vice-præfect of the island, was interpreted as an evidence of his own guilt; and the governor was urged to the necessity of turning against his breast the sword with which he had been provoked to wound the Imperial minister. The most innocent subjects of the West were exposed to exile and confiscation, to death and torture; and as the timid are always cruel, the mind of Constantius was inaccessible to mercy.