Battle of the Milvian Bridge: "In This Sign You Will Conquer;" Constantine Defeats Rival Emperor

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Battle of the Milvian Bridge: "In This Sign You Will Conquer;" Constantine Defeats Rival Emperor

Fresco of "Battle of the Milvian Bridge" by Giulio Romano, painted 1520-1524
One of a number of frescos located in the Hall of Constantine,
In the Apostolic Palace, Vatican City, Italy

Today in Military History: October 28, AD 312

[Today's post is an update to one originally published in 2009]

Today's "stroll through military history" highlights one of the lesser known battles of history, but one that pushed Western civilization into the Christian fold, thanks to the vision (or visions?) of the Roman emperor Constantine. 


By the 290's the ruling emperor Diocletian had decided the Roman Empire was too large for one man to rule. Therefore, he developed the Tetrarchy (Greek for "rulership by four"). He divided the empire into essentially four parts, and appointed three additional co-Caesars to administer the empire with him. When Diocletian stepped down in 305, he appointed Flavius Constantius (called "Chlorus" or "the Pale) to succeed him. Little did he suspect this would be the first scene in a Roman drama of civil war that would not finally end until 324.

In the western empire, the rulers were Constantius and Marcus Maximian; in the east, Gaius Galerius and Flavius Valerius Severus were in charge. Constantius was Augustus (the senior co-emperor) for about 14 months when he died in July, 306. Almost instantly, Constantius' troops declared his son, Constantine, to be the new Augustus Augustus. His chief rival was Maxentius, son of Maximian. Constantine's claim to Augustus was recognized by Galerius (who was now the senior emperor of the Romans), and Maxentius' claim was rejected. However, Maxentius gathered his forces and began maneuvering for position.

Map of the divisions of the Roman Empire under the Tetrarchy, c. AD 300; Image created by the Coppermine Photo Gallery, courtesy of Wikipedia
Map of the divisions of the Roman Empire under the Tetrarchy, c. AD 300
Image created by the Coppermine Photo Gallery, courtesy of Wikipedia

On October 28, 306 Maxentius entered Rome and was declared Augustus by the army. Severus then marched on Rome, seeking to evict the man most Romans considered a usurper. However, the majority of Severus' troops had served under Maximian, and promptly defected to Maxentius. Severus was captured and executed. Then, in the summer of 307, the eastern Augustus Galerius crossed into Italy to contend with Maxentius. He found nearly the entire Italian peninsula fortified against him, so he withdrew.

Prelude to the Battle

During the period of 307-312, various political and military maneuverings occurred, with Maxentius in command of Rome. Finally in early 312, Constantine resolved to end the rule of the usurper, and marched into Italy, heading for Rome with about a quarter of his total forces, about  45,000 men.

Constantine's army marched through Gaul (modern-day France) and traversed the always-tricky Alps to arrive in the northern areas of Italy. In an effort to slow down the invaders – and likely to whittle down their manpower – Maxentius threw four separate armies against Constantine. In each instance, Constantine's forces were victorious. After each fight, many of the defeated soldiers jointed the winning side. Therefore, Constantine's army likely numbered about 50,000 men as it approached the Empire's capital.

Maxentius was aware of Constantine's objective, and had prepared Rome for a long siege. However, when the omens were read by the Roman priests, they recommended that Maxentius meet Constantine in open battle. They especially recommended that he attack Constantine on October 28, 312, the anniversary of Maxentius' accession to Augustus, which they saw as a good omen. So, Maxentius gathered his forces and went to confront Constantine.

Maxentius marched out of one of the northern gates of the city of Rome (probably the Porta Fontinalis), along the Via Flaminia. His army apparently outnumbered Constantine's forces, numbering about 75,000 men. At one point, the road crossed the Tiber River by the Milvian Bridge. [A rebuilt Milvian Bridge still stands to this day.] After crossing the Tiber, Maxentius ordered a temporary but less-stable wooden bridge or a pontoon bridge (the ancient chronicles disagree) built near the main bridge. The stone bridge crossing the Tiber had been cut to prevent its use by Constantine‘s army. Maxentius deployed his army just beyond the bridge, apparently determined to defend Rome or die trying. The defending army marched along the Flaminian Way until it reached the village of Saxa Rubra, about nine miles north of Rome. The area boasted a large, flat plain, and Maxentius decided he would make his stand here.

"In this sign, you will conquer"

Late Roman infantry, displaying the Chi-Rho on shields and battle standard; Image courtesy of
Late Roman infantry, displaying the Chi-Rho on shields and battle standard
Image courtesy of

At this point, we will mention one of the reasons why this battle has achieved some fame among historians. Again, various historians of the time disagree on the exact facts. One chronicler, Lactantius, claims that the night before the battle, Constantine had a particularly vivid dream. In this vision, he saw a Latin cross with the top rounded to look like a P (the letter rho) . A voice in his dream directs him to paint this symbol on the shields of his soldiers, indicating that the Christian God's favor is with him and his army. [This phenomena is known as a "sun dog" which is caused by the sun's light reacting with ice crystals in the atmosphere.]

A 'sun dog,' photographed in Fargo, ND on February 18, 2009
A 'sun dog,' photographed in Fargo, ND on February 18, 2009

However, another historian, Eusebius, told a different story (Eusebius also claims that he heard this version from the mouth of Constantine himself). Sometime before the battle as his army was marching toward Rome, Constantine and many of the soldiers of his army saw a strange thing: a cross of light on the face of the sun, and in fiery lettering, "In this sign, you will conquer." [The phrase was in Greek, according to Eusebius, so for his audience he translated it into Latin as, "in hoc signo vinces."] Constantine did not understand the apparition, so later that night, Christ appeared to Constantine in a dream to explain the sign. According to Constantine, Christ said that he should inscribe the symbol on his army banners, as a sign that he had the favor of the Christian god.

Battle of the Milvian Bridge

Constantine's victorious soldiers (right) force Maxentian troops across pontoon bridge; Artist unknown, image courtesy of
Constantine's victorious soldiers (right) force Maxentian troops across pontoon bridge
Artist unknown, image courtesy of

Both armies spent a few hours deploying for battle. The two armies mirrored each other, with cavalry on the flanks and the infantry in between. As the defending army commander, Maxentius enjoyed the numerical advantage, and was able to equal the breadth of Constantine's line with unusually deep ranks. This was a common sign of unreliable troops since the back ranks would forestall those in the front from fleeing.

To open the battle, Constantine personally led his Germano-Gallic cavalry against the Maxentian horsemen – probably light Numidian and Moorish cavalry – with Constantine's heavier horsemen coming away victorious. With the rout of the enemy horse, Constantine's infantrymen charged into the less-experience front ranks of the defending force. Amazingly, these hastily-recruited troops performed fairly well, initially holding back the Constantinian troops. However, the northern Roman force began to push Maxentius' lines back toward the Tiber. Maxentius' Praetorian Guardsmen held their ground, performing well, but they too were eventually forced to give ground and died almost to the last man.

After several hours of hard, bloody combat, the Maxentian troops broke and routed, retreating across the makeshift bridge meant for the invading army. Before long, the bridge collapsed, trapping thousands of Maxentian troops on the north riverbank; many surrendered, those that did not were slaughtered. One chronicle states Maxentius himself tried to force his way through the tidal wave of his retreating army. He eventually drowned in the Tiber, one account said his horse threw him into the river.


Just as the number of participants In each army are speculative, the number of casualties are equally unknown. However it probably would not strain credulity to say that Maxentius' army was decimated. Those that were not killed or drowned surrendered to Constantine's men.

Footnote #1: The next day, Constantine entered the Eternal City to great public acclamation. The body of Maxentius was fished out of the Tiber, and promptly beheaded and paraded through the city's streets. Constantine then did something unheard of: he deliberately avoided the temples of the major Roman gods to give sacrifices for his victory, and instead when to the imperial palace. Later, he visited the Senate and promised to restore that body's ancient privileges.

The current-day Ponte Milvio (Milvian Bridge), crossing the Tiber River, outside Rome, Italy
The current-day Ponte Milvio (Milvian Bridge), crossing the Tiber River, outside Rome, Italy

Footnote #2: Constantine then instituted the damnatio memoriae, or the "damnation of memory." Constantine had any and all of Maxentius' legislation or imperial decrees nullified, and any building projects started by Maxentius were taken over by Constantine, and labeled instead as his projects. Constantine even extended the damnation to the laws and projects of Maximian (Maxentius' father). He also removed the greatest of Maxentius' supporters by disbanding the Praetorian Guard (any that had not perished on the battlefield) and the Imperial Horse Guard.

Footnote #3: In 313, Constantine issued the Edict of Milan, making it imperial policy that Christianity was a recognized religion in the Empire and immune from persecution. It did not, as some people believe, make Christianity the only religion of the Empire: it merely gave Christians – and for that matter, all other religions worshipped in the Empire – equal footing with the Old Gods. It was more legal document to ensure that Christians were free to worship however they pleased.

Footnote #4: Perhaps the one action for which Constantine is best known today is the creation of the city of Constantinople (now known as Istanbul). In 324 the Emperor selected the site of the city of Byzantium. It lay astride the land route from Europe to Asia and the seaway from the Black Sea to the Mediterranean. It also possessed, in the Golden Horn, an excellent and spacious harbor. In Greek and early Roman times, Byzantium was famous for its strategic geographic position that made it difficult to besiege and capture, and its position at the crossroads of the Asiatic-European trade route over land and as the gateway between the Mediterranean and Black seas made it too valuable a settlement to abandon. Constantine hired builders, mason, architects, engineers, and their associated workers to rebuild the city (it had been razed to the ground by Septimus Severus in 196 when it supported a rival emperor). It took 6 years to build Nova Roma (the "New Rome" as Constantine originally name it). The new capital city was consecrated on May 11, 330.

Constantinople in the Byzantine Period, c. 700-1453, image created by cplakidas; (Note the original Constantinian outer wall, the inner gray line in left-center of map)
Constantinople in the Byzantine Period, c. 700-1453, image created by cplakidas
(Note the original Constantinian outer wall, the inner gray line in left-center of map)

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