Civil wars of the Tetrarchy

• 1. Introduction

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  The civil wars of the Tetrarchy tore the Roman Empire apart across eighteen years, beginning in 306 AD with a throne seized at York and ending only when one man stood alone over the ruins of shared power. At stake was not merely who would rule, but whether the elaborate system of shared governance that had kept Rome together would survive at all. Diocletian had built his Tetrarchy to last, dividing authority between four emperors to prevent the chaos that had nearly destroyed Rome in the previous century. Within a year of his retirement, that system was fatally broken. What follows is the story of how ambition, loyalty, and one soldier's vision of a cross in the sky unraveled an empire's best attempt at political order.

• 2. Diocletian's Design

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  Diocletian instituted the Tetrarchy in 293 AD, responding to the wreckage left by the Crisis of the Third Century. He had already taken his first step toward shared rule years earlier: in 285, he designated the general Maximian as Caesar, the junior rank of emperor, then promoted him to the equal rank of Augustus in 286. Diocletian managed the eastern provinces; Maximian handled the west. In 293, both senior emperors appointed a Caesar each to assist them. Galerius went east under Diocletian, and Constantius Chlorus went west under Maximian. The system had four emperors locked into a hierarchy, each with a defined zone and a clear line of succession. When Diocletian and Maximian jointly abdicated in 305 AD, Constantius and Galerius rose to Augustus, and they in turn elevated two new Caesars: Severus in the west and Maximinus Daza in the east. On paper, the machine would run without its architect. In practice, it depended entirely on the men inside it choosing to honor its rules.

• 3. The First Cracks

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  Constantius died at Eburacum, the city now known as York, in 306 AD. His troops refused to wait for the system to operate on its own terms. Rather than allowing Severus to ascend from Caesar to Augustus as the Tetrarchy prescribed, the soldiers at Eburacum elevated Constantius' own son, Constantine, to the rank of Augustus on the spot. Galerius received a portrait of Constantine wearing a crown of laurels. He reportedly came close to setting it on fire. His advisers argued that rejection meant certain war, so Galerius made a calculated compromise: Constantine could hold the title of Caesar, while Severus took the Augustus rank. To make clear who held the power to legitimize emperors, Galerius personally dispatched the traditional purple robes to Constantine, who accepted the arrangement knowing it would quiet doubts about whether he belonged on the throne at all. That same year, Maxentius, son of the retired Maximian, read the situation differently. He declared himself Emperor at Rome in 306, calculating that if Constantine could seize power, so could he. Galerius ordered Severus to crush the usurpation quickly. Severus marched from his capital at Mediolanum toward Rome, but his army had previously served under Maximian, and when they arrived, they switched sides. Severus fled to Ravenna. In March or April 307, Maximian persuaded him to surrender in exchange for lenient treatment. Maximian's assurance proved worthless. Severus was displayed as a captive and imprisoned at Tres Tabernae, near Rome.

• 4. War of Constantine and Maxentius

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  Galerius' final act was a letter posted at Nicomedia on the 30th of April 311, proclaiming an end to the persecution of Christians. He died shortly afterward, removing what little stability remained in the Tetrarchy. Maxentius had been consolidating his position in Italy, but his support was corroding. Higher taxes and suppressed trade produced riots in Rome and Carthage. Domitius Alexander briefly seized Africa from him. By 312 he was, as the source records, barely tolerated rather than actively supported, even by Christian Italians he had tried to cultivate. In the summer of 311, Maxentius formally declared war on Constantine, vowing to avenge the death of his father Maximian, who had hanged himself at Massilia in July 310 after Constantine captured him following a failed rebellion. Constantine moved first and moved fast. In the spring of 312, he crossed the Cottian Alps with roughly 40,000 soldiers, a quarter of his total force. His army reached Segusium, a heavily fortified town that shut its gates. Constantine ordered the gates burned and the walls scaled. He took it quickly, then ordered his troops not to loot it. At Augusta Taurinorum, which is Turin today, Constantine met a large Maxentian cavalry force. His soldiers encircled the horsemen and dismounted them with iron-tipped clubs. Turin then opened its gates. He rested his army at Milan until mid-summer, then advanced through Brescia toward Verona, where Maxentius' praetorian prefect Ruricius Pompeianus commanded a large force sheltered by the Adige river on three sides. Constantine surrounded the town; Ruricius slipped out, gathered reinforcements, and returned. Constantine refused to lift the siege. In the fight that followed, Ruricius was killed. Verona surrendered, then Aquileia, Mutina, and Ravenna. The road to Rome lay open. Maxentius chose to meet Constantine in open battle on the 28th of October 312, the sixth anniversary of his own reign. He had consulted the Sibylline Books that morning and heard a prophecy that the enemy of the Romans would die that day. Constantine's army arrived at the field bearing the Chi-Rho on its shields. According to Lactantius, Constantine had dreamed the night before that he was told to mark the heavenly sign of God on his soldiers' shields. The battle was brief. Maxentius' troops broke before the first charge. The cavalry guards and praetorians initially held but then broke and fled to the river. Maxentius rode with them and was pushed by the press of fleeing bodies into the Tiber, where he drowned. Constantine entered Rome on the 29th of October to popular jubilation. Maxentius' body was pulled from the river, decapitated, and his head was paraded through the streets before being sent to Carthage, which immediately ceased all resistance.

• 5. Licinius Versus Maximinus Daza

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  After Galerius died in 311, the eastern half of the empire split between Licinius and Maximinus Daza. Daza resented that Galerius had ranked Licinius above him and watched for an opportunity to act. He found one in February 313, when he learned in Syria about the alliance Constantine and Licinius had sealed in Mediolanum. Daza left Syria with 70,000 men. The harsh weather along his route weakened his army severely by the time he reached Bithynia. He crossed the Bosporus in April 313, reached Byzantium, which was held by Licinius' troops, and took it after an eleven-day siege. He then seized Heraclea after a shorter siege. Licinius had mustered a much smaller force, possibly around 30,000 men, and reached Adrianople while Daza was still occupied at Heraclea. The two armies met on the 30th of April 313 at the Battle of Tzirallum. Daza's forces were crushed. He stripped off the imperial purple, dressed himself as a slave, and fled to Nicomedia. He tried to stop Licinius' advance by fortifying the Cilician Gates, but Licinius broke through, forcing Daza back to Tarsus. The war ended with Daza's death in August 313, leaving Licinius in control of the entire eastern half of the empire.

• 6. Constantine and Licinius

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  In 313, Constantine and Licinius met in Milan to solidify their alliance with the marriage of Licinius to Constantine's half-sister Constantia. At this meeting they agreed on the Edict of Milan, granting full tolerance to Christianity and all religions across the empire. The document went beyond tolerance. It legalized Christianity, restored property seized during Diocletian's persecutions, and repudiated religious coercion in explicit terms. Its language referred to the divine only through general phrases such as "Divinity" and "Supreme Divinity" to avoid imposing a specific theology. The conference ended abruptly when news arrived that Maximinus had crossed the Bosporus. Licinius departed to fight him, which he did successfully. Relations between Constantine and Licinius then deteriorated steadily. In 316, Constantine appointed his brother-in-law Bassianus as Caesar. Bassianus was subsequently discovered to be conspiring against Constantine, possibly at the urging of his own brother Senecio, who was closely tied to Licinius. Constantine demanded Licinius hand Senecio over. Licinius refused. The first war between them ended with Constantine victorious at Cibalae. They clashed again at the Battle of Campus Ardiensis in 317, then settled on terms: Constantine's sons Crispus and Constantine II, along with Licinius' son Licinianus, were all named Caesars. The peace held for several years. In 320, Licinius reversed course on the Edict of Milan and resumed persecution of Christians. By 324, the final conflict was framed on both sides in explicitly religious terms. Licinius, supported by Goth mercenaries, was seen as the defender of the old Pagan order. Constantine marched under the labarum, the standard derived from his vision before the Milvian Bridge. Constantine won the Battle of Adrianople. Licinius fled across the Bosphorus and appointed Martius Martinianus, the commander of his own bodyguard, as Caesar. Constantine then won the Battle of the Hellespont and, finally, the Battle of Chrysopolis on the 18th of September 324. Licinius and Martinianus surrendered at Nicomedia under a promise that their lives would be spared. They were sent to live as private citizens, Licinius in Thessalonica and Martinianus in Cappadocia. In 325, Constantine accused Licinius of plotting against him. Both men were arrested and hanged. Licinius' son, who was also the son of Constantine's own half-sister, was executed as well. Constantine was now the sole emperor of the Roman Empire.