Its location at the north end of the Campus Martius ensured that it would always have military significance. There were open areas on both sides of the Tiber. The Campus Martius, on the city side of the bridge was a place where the Romans could muster defending troops or assemble expeditionary forces. Before urban development took over the Campus Martius, it was the place where Roman legions drilled and where returning victorious generals encamped their troops while waiting for the Senate to decide on whether a formal Victory would be celebrated. On the other side of the River, north of the Ponte Milvio, was an area in close proximity to Rome where an invading force could maneuvre and regroup before trying to fight its way into the city. So the Ponte Milvio was always a military bridge. And it was tested many times.
A Battle at the bridge: Long before the famous battle between Constantine and Maxentius, there was another battle of great import at the Milvian Bridge, but to understand it, we need to know a little of the background. Lucius Cornelius Sulla had won the first full-scale civil war in Roman history in 82 BC and ruled as dictator for three years. Once in power he passed laws transferring power from the Tribunes back to the Senate. Although that may seem like democratization, in fact it was an attempt to prolong the power of a decadent, elite, Patrician class in the Senate at the expense of the people, whose spokesmen were the Tribunes. At the end of his dictatorship in 79 BC, Sulla surprised everyone by retiring to write his memoirs.
Sulla's "reforms" were immediately challenged. A Senator named Marcus Aemilius Lepidus, who had wide backing among the people, was elected Consul for the year 78 BC with the help of Pompey, and, when Sulla died that same year, Lepidus proposed legislation rescinding Sulla's laws. The Senate, not surprisingly, rejected his proposals. Lepidus took up arms, gathered forces in the north, and marched on Rome. He got as far as the Milvian Bridge where Quintus Lutatius Catalus repelled him. By holding the bridge, Catalus ensured that the senatorial party stayed in power. The attack by Lepidus was one of the circumstances that moved Pompey into the senatorial camp. Pompey, who was outraged by Lepidus' unconstitutional moves, led out an army that decisively defeated Lepidus in Etruria. This was the same Pompey who was the leader of the Senatorial party and virtual dictator in Rome when Caesar crossed the Rubicon in 49 BC. But when Caesar arrived at the Milvian Bridge he found it undefended -- Pompey and his crowd had fled south in panic.
The Cataline Conspiracy: Between the repulsion of Lepidus by Catalus and the uneventful passage of Caesar over the bridge, there were 29 years of almost uninterrupted trouble as the Roman republic collapse in on itself. One of the most intriguing events of this chaotic period was the Conspiracy of Cataline (not the same guy as Catalus), and again the Milvian Bridge figures into the story.
All of the accounts of Cataline were written after his conspiracy was discovered and his revolt was put down, so what was said about him by his victorious enemies might be suspect. But even his former followers tell the same story: he was eloquent, charming, rich, dissolute, mean, extravagant, unwise, and insatiably ambitious. He was the epitome of the chaos of his times. Cataline surrounded himself (as did many others of the nobility, and as do boxers, football players, and rap stars of today) with an entourage of thugs and criminals. They and Cataline were accused by their enemies (and later by some of their followers) of almost any conceivable excess. There is no doubt that they were a criminal gang. Their power in the city quickly grew and they eventually got the support of a number of Senators, most of whom were enemies of Pompey -- they appear to have thought that they could use Cataline to bring Pompey down a notch or two. Some even said that Crassus, Pompey's chief rival, at least tacitly supported Cataline, who was plotting a coup.
In 64 BC, Pompey was off fighting the Mithradatic Wars overseas, and Cataline saw a chance to put his plan into action. But another schemer, Cicero, was also around, and he wanted to thwart Cataline. The goal of Cicero, Rome's most successful lawyer and most famous orator, had always been to avoid being identified with any of the political camps or gangs in the city -- he wanted to know and control everything, but to be seen as a "clean hands" outsider. He had spies planted everywhere, and some of them were in Cataline's gang. When he got wind of Cataline's impending coup attempt, Cicero bribed some of Cataline's supposed allies, barbarian representatives of the Allobroges tribe from Gaul, to help him trap Cataline. The details are complex, but, in short, Cicero's men and the magistrates intercepted the Allobroges along with Cataline's emissaries at (where else?) the Milvian Bridge as they were leaving town. There was a short scuffle -- the Allobroges didn't participate -- and then Cataline's men were captured. The Allobroges, acting on Cicero's suggestion, had asked the emissaries to bring along letters from Cataline outlining the plot, and these letters were taken and read out in the Senate by Cicero. The Senate was outraged, but more importantly, the general public, on whose support Cataline had depended, were also outraged, and this because of two things. First, although almost anything else might be tolerated, nobody should ever recruit barbarians, and especially Gauls, in a conspiracy against Rome. Secondly and more importantly, the letters seized from Cataline's emissaries made it clear that his plot involved widespread arson in the city, and fire was every Roman's worst nightmare.
Cataline escaped the but he and many of his high ranking followers were quickly caught and executed under quasi-legal circumstances. Accusations against Crassus were suppressed, either because the Senate thought he was too powerful to beard or because, as Crassus maintained, the accusations were trumped up by his enemy Cicero. Cicero was said to have resisted attempts by his allies to bring trumped up charges against Julius Caesar.
Constantine and Maxentius at the Bridge: This was the main event at the Milvian Bridge as far as Christianity and "Western Civilization" was concerned. You could say, however, that from another viewpoint it spelled the end of ancient Rome. The Roman Empire was already on the slippery slope to its "decline and fall", but Constantine clearly gave it another downhill push.
Prelude to the battle: Diocletian, realizing that the Empire was too big to rule alone, appointed a co-emperor and in 305 AD the two emperors, Diocletian and Maximian, abdicated, to be succeeded by their respective deputy emperors, Galerius and Constantius. Two new deputy emperors were appointed, Galerius Valerius Maximinus in the East and Flavius Valerius Severus in the West. Constantine, the son of Constantius, was passed over. Constantine made his way through the territories of the hostile Severus to join his father in France. They crossed together to Britain and fought a campaign in the north before Constantius' death at Eboracum (modern York) in 306. Constantine was immediately acclaimed emperor by the army and then engaged in a complex series of civil wars. Maxentius, the son of Maximian, had rebelled and taken over Rome. Maxentius originally had help from Maximian, but there was a family tiff and Maximian joined Constantine in Gaul. Maximian later betrayed Constantine and was murdered or forced to commit suicide (310). Constantine, married Maximian's daughter Fausta and invaded Italy in 312.
After a lightning campaign, Constantine defeated the army of his now brother-in-law, Maxentius, at Saxa Rubra nine miles north of the Milvian Bridge. The remnant of Maxentius' army was caught while they were franticly trying to reach the safety of Rome's walls by crossing the Milvian Bridge. The bridge, according to most accounts, collapsed under the weight of Maxentius' remainders, and Maxentius, weighted down by his armor, drowned in the Tiber. It took another twelve years for Constantine to also gain control of the eastern half of the empire and become the sole emperor.
Just another struggle for power in Rome, but this one was decided, Constantine said, by the intervention of a foreign god, Christ. Constantine said he had two visions before the battle at Saxa Rubra and pursuit of Maxentius to the Milvian Bridge. In the first, an image appeared in the sky that looked like the "chi" and "rho", the first two letters of the name of Christ as written in Greek (It looks like the capital "P" superimposed on a capital 'X" in the English alphabet.) Under the image floated a banner, which had written on it in Greek, "Conquer with this sign." Constantine that night had a second vision, a dream, in which, he said, Christ told him to put the same image on the tunics and shields of his soldiers to ensure victory. He also was said to have put the image on his imperial standard after removing the imperial eagle and replacing it with a crown for Christ (the same crown we now see surmounting SPQR signs throughout Rome.)
Theologians and realists have taken predictable positions on the veracity of Constantine's visions, but the fact remains that he did win and that, shortly thereafter, he legalized Christianity and outlawed Mithraism, Christainity's only real competition for the religious zeal of the Romans, who were deserting the old gods and embracing "eastern" religions en masse.
Unfortunately for Rome, Constantine decided that Rome was no longer the place from which to rule the empire. Within three months of his victory over Maxentius, he decamped and eventually moved his capital to a new city at the juncture of Europe and Asia and called it, with characteristic Roman modesty, Constantinople. He returned only twice, to celebrate the tenth and twentieth anniversaries of his victory. When the Imperium left Rome, everything else did too. Population rapidly dwindled from about 1.3 million in Constantine's Rome to less than 50 thousand 800 years later. Thus Constantine's strong push of Rome down the slippery slope.
The Milvian Bridge, of course, survived Constantine's abandonment, and Rome revived, due largely to the efforts of the Popes. Returning to the city in the 14th century, after the Avignon Captivity, they rebuilt the city and its bridges -- the Milvian Bridge had been damaged in 538 when Belisarius had fought the Visigoths for control and again in a local war between the Orsini and the Colonna in 1335. Papal reconstruction was accomplished using funds raised by Jubilee celebrations that were held first every 50 years and eventually, until today, every 25 years. For centuries it was the approved entry into Rome for Jubilee pilgrims. Garibaldi and his men did serious damage to the bridge in two battles on April 30 and May 30, 1849 -- they were trying to keep the French from crossing -- but it was again restored after the 1870 Reunification. It carried vehicular traffic until 1956 when it was declared a national treasure, and since then only pedestrians have been allowed. There were also floods and other natural disasters, but, for the most part, what you see today is the ancient Roman structure.
Considering its historical importance, the Ponte Milvio gets surprisingly little attention and appears to get no promotion at all as a tourist attraction. All Romans know its location and at least parts of its history. Cross it. Charlemagne did 1201 years ago in 799 AD.
Ordering information for the best ever English Language book on Roman Bridges: $110.00
Milvian Bridge time-line (in Italian)
Roman Historian Sallust on the Cataline Conspiracy -- the whole exciting and complex story as told by an insider
Account of the Constantine/Maxentius battle in Gibbon's Decline and Fall of the Roman Empire
The Arch of Constantine -- his own monument to his own victory at the Milvian Bridge