In late October 312 CE, the fate of the future of the Roman world was decided near the Pons Milvius, the Milvian Bridge (the modern-day Ponte Milvio, Italy), crossing the River Tiber some three miles (5km) north of Rome on the via Flaminia. The battle was the culmination of the war between rival Roman emperors, Maxentius and Constantine, to see who would dominate the western Roman Empire.
In 293 CE, Emperor Diocletian had introduced a system of two emperors (augusti; singular: augustus) and two junior-emperors (caesares; singular: caesar), to govern what had become an unstable system of one emperor ruling over the entire empire. The previous century had seen a great deal of instability and more than 50 emperors and usurpers. This new system was known as the Tetrarchy and saw the empire split between east and west. The west was governed from Mediolanum (modern-day Milan) and Augusta Treverorum (modern-day Trier, Germany); the east from Nicomedia (modern-day Izmit, Turkey) and Sirmium (modern-day Sremska Mitrovica, near Belgrade, Serbia). The system worked well until 305 CE when Diocletian and his co-emperor, Maximian, retired and the caesares, Galerius and Constantius, were raised to augusti and new caesares were appointed: Maximinus Daza to Galerius and Valerius Severus to Constantius.
Constantius died in York in July 306 CE, and his son, Flavius Valerius Constantius (known as Constantine), was acclaimed augustus by his father’s army. Constantius’ co-emperor Galerius promoted Severus to augustus as the system dictated. In October that year, the retired emperor Maximian’s son, Marcus Aurelius Valerius Maxentius, declared himself augustus (in opposition to Severus). Rivalry and ambition were already putting the Tetrarchy system at risk.
Maxentius had probably expected to succeed his father, but in this he was disappointed – he would, however, prove to be popular with the troops as his father’s son. Severus, sent to defeat Maxentius by Galerius, suffered the ignominy of his army deserting to his enemy and he surrendered to Maxentius in 307 CE (he was later murdered). Galerius marched on Rome himself but he, too, was unsuccessful. Both Maxentius and Constantine were therefore recognised as augusti. Both men had had military careers which would prove immensely useful to their claims.
A crisis meeting to avoid further civil war held in Carnuntum (near modern-day Petronell-Carnuntum and Bad Deutsch-Altenburg, Austria) in 308 CE declared that Licinius would be augustus of the west with Constantine as his caesar, Galerius would be augustus of
