After the death of Galerius in 311 AD, the Roman Empire was split between four Augustii – Constantine I in the western provinces allied with Licinius I in the Balkans, Pannoniae and Illyria and Maxentius controlling Italy and Africa, allied with Maximinus II Daia in Asia and Egypt. Each of the four viewed the others as hostile, so the alliances that did exist were out of convenience to choose sides for the inevitable civil war.
Constantine invaded Italy to confront Maxentius, even though he was vastly outnumbered militarily. What he had on his side was discipline and tactics. The armies of Maxentius had many veterans who were enjoying the finer things in Rome after having served their emperor. The younger recruits were not yet fully trained. Constantine, however, had well-seasoned troops and employed battle strategies that confused his opponents.
The 400-mile march from Milan on the Via Aemelia to Ariminum, continuing on the Via Flaminia to Rome, gave the Praetorian Guard ample time to convince Maxentius the threat headed their way was real. Constantine had routed many of Maxentius’s elite at Turin, and entered Milan victorious. The march continued on to Brescia, to deal with Pompaeianus, who commanded the armies of Verona and the surrounding area. Constantine defeated that army, but Pompaeianus fled to Verona, with its capable defenses and strategic Adidge River on three sides. Undaunted, Constantine laid siege and was able to take it and chase Pompaeianus out during fierce night battles.
Constantine and his troops continued, having captured Segusio, Turin, Milan, Verona, Brescia and Aquileia, securing all of northern Italy. Meanwhile, agents of Constantine were busy undermining support for Maxentius in Rome. Even with the subterfuge, Maxentius was able to raise an army of 170,000 troops, led by the Praetorian Guards, and had already built up the defenses of the city walls and added temporary platforms to the Milvian Bridge for troop movements.
The armies of Constantine arrived on the outskirts of Rome, preparing for the pitched battle that would ensue. The night before the battle it is said Constantine was visited by an angel, who said “Hoc Signo Victor Eris”, or “In this sign, ye shall conquer”, and was shown the Chi-Rho, or Christogram. Constantine ordered his troops to paint the symbol on their shields and prepare to go into battle under the protection of God.
The next morning, October 28, 312, exactly six years after proclaiming himself emperor in Rome, Maxentius led his army to face Constantine. Constantine’s scouts reported Maxentius was advancing in a long column, with rear defenses at the Tiber River and Milvian Bridge. Constantine allowed the army to advance to draw them in, then flanked with light cavalry, causing the front of the army to try to break and retreat. The Praetorians refused to give up and most were killed throughout the day. With so much of the main army retreating, the temporary structures created to allow for the troops to pass over the Tiber River eventually failed and Maxentius himself fell into the river and drowned from the weight of his armor. Constantine marched into the city and was welcomed as the emperor of the Western Empire.