Civil wars of the Tetrarchy
In 293, feeling more focus was needed on both civic and military problems, Diocletian, with Maximian's consent, expanded the imperial college by appointing two Caesars (one responsible to each Augustus) – Galerius and Constantius Chlorus.
The senior emperors jointly abdicated and retired in 305 AD, allowing Constantius and Galerius to be elevated in rank to Augusti.
Abandoned, Severus fled to Ravenna, an impregnable position: in March or April 307, Maximian convinced him to surrender in exchange for leniency.
The joint rule of Maxentius and Maximian in Rome was tested further when Galerius himself marched to Italy in the summer of 307 with an even larger army.
The usurpers' strong position in Italy prompted Galerius to open negotiations, during which Maxentius repeated the strategy he had used with Severus: attracted by the promise of large sums of money and the authority of Maximian, many soldiers defected from the invasion army.
In the conference of Carnuntum in the autumn of 308, Maxentius was once again denied recognition as legitimate emperor, and Licinius was appointed Augustus with the task of regaining the usurper's domain.
He offered bribes to any soldiers who would support him as emperor, but most of Constantine's army remained loyal and Maximian was compelled to leave.
Constantine soon heard of the rebellion, abandoned his campaign against the Franks, and moved quickly to southern Gaul, where he confronted the fleeing Maximian at Massilia.
The town was better able to withstand a longer siege than Arles, but it made little difference as loyal citizens opened the rear gates to Constantine.
[19][20][14][21][22][23] His final act survives: a letter to the provincials posted in Nicomedia on 30 April 311, proclaiming an end to the persecutions of Christians, and the resumption of religious toleration.
[33] He fortified northern Italy, and strengthened his support in the Christian community by allowing it to elect a new Bishop of Rome, Eusebius.
His early support dissolved in the wake of heightened tax rates and depressed trade; riots broke out in Rome and Carthage;[37][38][21][39] and Domitius Alexander was able to briefly usurp his authority in Africa.
[44] Constantine's advisers and generals cautioned against preemptive attack on Maxentius;[30][45] even his soothsayers recommended against it, stating that the sacrifices had produced unfavourable omens.
[46][47] Constantine, with a spirit that left a deep impression on his followers, inspiring some to believe that he had some form of supernatural guidance,[47] ignored all these cautions.
[30][49] At the approach to the west of the important city of Augusta Taurinorum (Turin, Italy), Constantine encountered a large force of heavily armed Maxentian cavalry.
[56][45][59] Ruricius Pompeianus, general of the Veronese forces and Maxentius' praetorian prefect,[56][59] was in a strong defensive position, since the town was surrounded on three sides by the Adige.
[60][56][63] He still controlled Rome's praetorian guards, was well-stocked with African grain, and was surrounded on all sides by the seemingly impregnable Aurelian Walls.
He ordered all bridges across the Tiber cut, reportedly on the counsel of the gods,[56] and left the rest of central Italy undefended; Constantine secured that region's support without challenge.
[60][47][66][67] Maxentius, no longer certain that he would emerge from a siege victorious, built a temporary boat bridge across the Tiber in preparation for a field battle against Constantine.
[71][72][47][73][70] Maxentius organized his forces – still twice the size of Constantine's – in long lines facing the battle plain, with their backs to the river.
"[76] Eusebius describes another version, where, while marching at midday, "he saw with his own eyes in the heavens a trophy of the cross arising from the light of the sun, carrying the message, In Hoc Signo Vinces or 'Conquer By This'";[77][72][78][63] in Eusebius's account, Constantine had a dream the following night, in which Christ appeared with the same heavenly sign, and told him to make a standard, the labarum, for his army in that form.
[83] Eusebius describes the sign as Chi (Χ) traversed by Rho (Ρ), or ☧ a symbol representing the first two letters of the Greek spelling of the word Christos or Christ.
Maxentius rode with them, and attempted to cross the bridge of boats, but he was pushed by the mass of his fleeing soldiers into the Tiber, and drowned.
Deciding to take the initiative, Daza left Syria with 70,000 men and reached Bithynia, although harsh weather he encountered along the way had gravely weakened his army.
Believing he still had a chance to come out victorious, Daza attempted to stop the advance of Licinius at the Cilician Gates by establishing fortifications there.
During this meeting, the emperors agreed on the so-called Edict of Milan,[a] officially granting full tolerance to "Christianity and all" religions in the Empire.
[104] The document had special benefits for Christians, legalizing their religion and granting them restoration for all property seized during Diocletian's persecution.
[105] The conference was cut short, however, when news reached Licinius that his rival Maximinus had crossed the Bosporus and invaded European territory.
Licinius departed and eventually defeated Maximinus, gaining control over the entire eastern half of the Roman Empire.
