The half-century after the assassination of the emperor Alexander Severus by his own troops, which took place in 235 AD, was an era of prolonged crisis for the Roman Empire. It is often described as a “military anarchy”, with one general after another contending for the throne, and most emperors meeting a violent death at the hands of their successors after only a few years. The Empire also saw various barbarian invasions, severe economic instability, and a significant plague that lasted for 13 years. By the end of the 260s, it had broken into three separate states; these were reunited by the brief but highly effective reign of the emperor Aurelian (270-5), who was murdered after five years and several astonishingly successful military campaigns.
The man who finally began to restore stability was Diocletian, who became emperor in 284, and is now infamous as the last major persecutor of the Christians. Whatever else his faults, he had the genius to recognize that the empire was too large for a single man to rule, and needed to establish an orderly succession. He therefore created the system known as the Tetrarchy, by which the Empire was divided into two parts, East and West, each ruled by an emperor and a co-emperor, respectively titled “Augustus” and “Caesar.” After twenty years, each Augustus would resign and be succeeded by his Caesar; if he died first, the Caesar would finish his Augustus’ term and appoint a successor.
No one will be surprised to read that this system did not last long beyond the first peaceful transition. When the two Augusti, Diocletian and Maximian, resigned in 305, Constantius Chlorus became Augustus of the West, with a Caesar named Valerius Severus, and Galerius of the East, with his Caesar Maximin Daia. Chlorus died in Gaul the following year, and his troops refused to recognize anyone other than his son Constantine as emperor, leading to another round of civil war.
(Constantine’s Vision of the Cross, depicted in the Room of Constantine within the so-called Stanze of Raphael, now part of the Vatican Museums; 1520-24, by the students of Raphael. Public domain image from Wikimedia Commons.)
The struggle that ensued is an extremely complicated matter; suffice it to say here that over the next six years, Italy and Africa recognized a usurper named Maxentius as Augustus of the West. By 312, Constantine had consolidated his position sufficiently well to invade Italy and assert his claim as emperor. Over the summer and early fall, he defeated the forces which supported Maxentius in several places in northern Italy, and by October, was ready to march towards Rome. Maxentius fortified the city, which was essentially impossible to besiege, surrounded by 19 kilometers of very high and very new walls, with an unstoppable water supply running through it, and a very large store of grain. Many sieges of great cities have ended with the besiegers themselves starving, and this could well have turned out to be one of them.
But Maxentius inexplicably decided to take his troops out of Rome, and meet Constantine near the Milvian Bridge, a bit more than a mile and a half north of the Flaminian Gate. He then positioned his troops with the Tiber behind them, leaving them too little room to maneuver, and his opponents smaller force was basically able push them into the river. This would pave the way for Constantine to end the Tetrarchy and become sole master of the Empire.
The Christian writer Lactantius offers this account of the famous vision which Constantine had on the eve of the battle, October 27, which place himself under the protection of the Christian God. Less than four months later, Constantine would issue the Edict of Milan, which ended the official persecution of the Church within the Empire.
“Iam mota inter eos fuerant arma civilia. Et quamvis se Maxentius Romae contineret, quod responsum acceperat periturum esse, si extra portas urbis exisset, tamen bellum per idoneos duces gerebatur. … Dimicatum, et Maxentiani milites praevalebant, donec postea confirmato animo Constantinus et ad utrumque paratus copias omnes ad urbem propius admovit et a regione pontis Mulvii consedit. … Commonitus est in quiete Constantinus, ut caeleste signum Dei notaret in scutis atque ita proelium committeret. Facit ut iussus est et transversa X littera, summo capite circumflexo, Christum in scutis notat. Quo signo armatus exercitus capit ferrum. … manus Dei supererat aciei. Maxentianus proterretur, ipse in fugam versus properat ad pontem, qui interruptus erat, ac multitudine fugientium pressus in Tiberim deturbatur.
And now a civil war broke out between them, and although Maxentius kept himself within Rome, because he had received a prophecy that he would perish if he went outside the city gates, nonetheless, the war was conducted by able generals. … They fought, and Maxentius’ soldiers prevailed, until Constantine, with steady mind, and prepared for every event, moved all his forces closer to the city, and encamped them by the Milvian bridge. … Constantine was advised in a dream to mark the heavenly sign of God on the shields (of his soldiers), and thus join the battle. He did as he was commanded, and he marked Christ on their shields with the letter Χ, with the top of the upper stroke bent back (to form the Greek letter rho). Armed with this sign (ΧΡ), his army took up its weapons. … the hand of the Lord prevailed in the battle. Maxentius was routed; he fled towards the bridge, which had been broken, and being driven by the multitude of fleeing men, he was driven headlong into the Tiber.” (On the Deaths of the Persecutors, 44)