Emperor Constantin's auspicious victory in the Battle of the Milvian Bridge in 312

28 October 312 near Rome, Emperor Constantine won his auspicious victory over the usurper Maxentius.
“Maxentius … called some senators together, ordered the Sibylline books to be searched. In them it was found that:—   “On the same day the enemy of the Romans should perish.” Led by this response to the hopes of victory, he went to the field.” (Lanctantius “Of the Manner in Which the Persecutors Died“)

An illustration of Emperor Constantine and his troops by Angus McBride* 

The twilight of the once mighty Praetorian Guard had already begun long since, when Emperor Maxentius revived the illustrious formation in Rome in the year of 306 CE. His predecessor Diocletian’s reform of the empire and relocation of the imperial administration centres away from the old capital to Nicomedia in modern western Turkey, Milan, Antioch, and Trier meant a radical decimation of the emperor’s traditional life guards regiment as well and the men who once made and murdered emperors and once had auctioned off the empire, back in 193 CE, were reduced in numbers, status and influence to a minor garrison in Rome. Maxentius, however, was an usurper and when the other legitimate emperors, after all there were four of them, regularly, after Diocletian’s reforms, ordered the Praetorian Guard to be dissolved, Severus, the local Caesar in Milan, was killed during the following riots and Maxentius had his cadre of loyal followers.

Sir Lawrence Alma-Tadema's imagination of the Praetorian Guard at the height of their power: "Proclaiming Claudius Emperor" from 1867

The proclamation of Constantine as Emperor of the West by his troops after the death of his father, one of the four original tetrarchs, wasn’t exactly legal either. For a couple of years, though, both he and Maxentius acted as Emperors of the West until they agreed that there can be only one and in the spring of 312, Constantine marched into Italy. A hopeless endeavour at first glance, since Constantine had to leave most of his troops along the Rhine to guard Gaul against the Germanic tribes and Maxentius could ride out the matter in a position of all-round defence in Rome. However, according to legend, Constantine had a dream that he had "to mark the heavenly sign of God on the shields of his soldiers... by means of a slanted letter X with the top of its head bent round, he marked Christ on their shields." and for inexplicable reasons, Maxentius left his more or less secure home base in Rome, marched to meet Constantine, tried to bottle him up north along the river Tiber near the old Milvian Bridge, a complicated manoeuvre that included a withdrawal of his centre to pull Constantine into an encirclement, the action was misunderstood, Maxentius’ men panicked, Constantine pursued, the usurper and his Praetorian guardsmen made a desperate last stand on the river, they were cut up and the battle was lost. Maxentius drowned while he tried to escape across a pontoon bridge.

A probably contemporary image of the battle's climax from the Arch of Constantine in Rome

According to some, Constantine and his men saw a cross shining over the battlefield before hostilities commenced on that day, along with the words "Εν Τούτῳ Νίκα", "In this sign, you shall conquer;" and later tradition made Constantine’s march along the Via Flaminia towards Rome something of a Road to Damascus, having the emperor converted to the true faith and whatnot but actually, no signs of a cross or a rho-chi  or something similar appear on the Arch of Constantine that was erected to celebrate his victory at the Milvian Bridge and Constantine was baptised on his death bed 25 years later, if at all. All sources agree, though, that after the battle, the Praetorian Guard was disbanded for good, their barracks, the Castra Praetoria, were torn down and 300 years of Imperial tradition, weal and woe, ended. A grand gesture by emperor Constantine, who soon became the sole emperor, far more tangible than an ominous vision and a certain sign that a new age had indeed begun.

*(http://en.wikipedia.org/wiki/Angus_McBride) found (and licensed) on http://www.lookandlearn.com/history-images/index.php
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