3-5. The Decisive Battle of Mursa
Marching against one usurper in the West, Constantius was anxious to secure the East to the dynasty of Constantine: the recent success of Lucillianus may have appeared dangerously complete. The Emperor’s nephew Gallus had, it would seem, for some time followed the Court, and while at Sirmium Constantius determined to create him Caesar. At the same time (15 March 351) his name was changed into Flavins Claudius Constantius, he was married to Constantia and became frater Augusti, forthwith the prince and his wife started for Antioch. Meanwhile Magnentius had not been idle; he had raised huge sums of money in Gaul, while Franks, Saxons, and Germans trooped to the support of their fellow-countryman, whose army now outnumbered that of Constantius. The latter however took the offensive in the spring of 351 and uniting Vetranio's troops with his own marched towards the Alpine passes. An ambush of Magnentius posted in the defiles of Atrans inflicted severe loss on his advance guard and the Emperor was compelled to withdraw. Elated by this success, the usurper now occupied Pannonia and passing Poetovio made for Sirmium.
Throughout his reign the policy of Constantius was marked by an anxious desire to husband the military forces of the Empire, and even now he was ready to compromise and to avoid the fearful struggle between the armies of Gaul and Illyricum. He dispatched Philippus, offering to acknowledge Magnentius as co- Augustus in the West if he would abandon any claim to Italy. The ambassador was detained, but his proposals after some delay rejected; the usurper was so certain of victory that his envoy the Senator Titianus could even counsel Constantius to abdicate.
An attack of Magnentius on Siscia was repulsed and an effort to cross the Save was also unsuccessful. Constantius then retired, preferring to await the enemy in open country where he could turn to the best advantage his superiority in cavalry. At Cibalae the army took up an entrenched position, while Magnentius advanced on Sirmium, hoping to meet with no resistance. Foiled in this he marched to Mursa in the rear of Constantius’s army. The latter was forced to relieve the town and here on 28 September the decisive battle was fought.
Behind Constantius flowed the Danube and on his right the Drave: for him flight must mean destruction. On both wings he posted mounted archers and in the forefront the mailed cavalry (cataphracti) which he had himself raised after the Persian model; in the centre the heavy armed infantry were stationed and in the rear the bowmen and slingers. Before the struggle Silvanus with his horsemen deserted Magnentius. From late afternoon till far into the night the battle raged; the cavalry of Constantius routed the enemy's right wing and this drew the whole line into confusion. Magnentius fled but Marcellinus continued the fight; the Gauls refused to acknowledge defeat; some few escaped through the darkness, but thousands were driven into the river or cut down upon the plain. It is said that Magnentius lost 24,000 men, Constantius 30,000.
The usurper took refuge in Aquileia and garrisoned the passes of the Alps; although his overtures were rejected, and though his schemes to murder the Caesar Gallus and thus to raise difficulties for Constantius in the East were foiled, yet the exhaustion of his enemies and the approach of winter made pursuit impossible. Constantius forthwith proclaimed an amnesty for all the adherents of Magnentius except only those immediately implicated in his brother's murder; many deserted the pretender and escaped by sea to the victor.
In the following year (352), Constantius forced the passes of the Julian Alps, while his fleet dominated the Po, Sicily, and Africa. At the news Magnentius fled to Gaul and by November the Emperor was already in Milan, abrogating all the fugitive's measures. In 353 Constantius crossed the Cottian Alps and at length, three years and a half after his assumption of the purple, Magnentius was surrounded in Lyons by his own troops, and finding his cause hopeless committed suicide, while his Caesar Decentius also perished by his own hand.
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