Civil wars of the Tetrarchy
Roman emperor Diocletian instituted the Tetrarchy in 293 AD to end the Crisis of the Third Century. This administrative division split imperial power between two Augusti and two Caesars. The first phase, known as the Diarchy, began when Diocletian appointed general Maximian as co-emperor. Maximian started as Caesar in 285 before his promotion to Augustus in 286. Diocletian managed Eastern regions while Maximian handled Western regions. In 293, both senior emperors expanded the college by appointing Galerius and Constantius Chlorus as new Caesars. Each Caesar was responsible to one Augustus. The system aimed to address civic and military problems with greater focus. Senior emperors jointly abdicated and retired in 305 AD. This allowed Constantius and Galerius to be elevated to Augusti. They then appointed Severus in the west under Constantius and Maximinus Daza in the east under Galerius.
The death of Constantius at Eburacum in 306 AD created the first crack in the political edifice. Troops at Eburacum elevated Constantius' son, Constantine, to the position of Augustus instead of accepting Severus. Galerius received a portrait of Constantine wearing a crown of laurels. Accepting this symbol would acknowledge Constantine as heir to his father's throne. Constantine claimed the army forced him into the role. Galerius almost set the portrait on fire but was calmed by advisers who argued denial meant certain war. He compromised by granting Constantine the title Caesar rather than Augustus. Severus received the latter office. Galerius personally sent purple robes to make it clear he alone gave legitimacy. Maxentius, son of Maximian, declared himself Emperor at Rome in 306 AD. Galerius ordered Severus to put down the usurpation. Severus moved from Mediolanum towards Rome where Maximian had come out of retirement. Soldiers switched sides to support their old commander Maximian. Severus fled to Ravenna. In March or April 307, Maximian convinced Severus to surrender for leniency. Despite assurances, Severus was displayed as a captive and imprisoned at Tres Tabernae near Rome.
The joint rule of Maxentius and Maximian faced further testing when Galerius marched to Italy in summer 307 with a larger army. During the invasion, Severus was put to death by Maxentius, likely at Tres Tabernae. The exact circumstances remain uncertain. Galerius withdrew after soldiers defected from his invasion army. Maxentius' reign over Italy and Africa became firmly established. Beginning in 307, he tried to arrange friendly contacts with Constantine. In summer that year, Maximian traveled to Gaul where Constantine married his daughter Fausta. Constantine received appointment as Augustus from the senior emperor. He avoided breaking with Galerius and did not openly support Maxentius during the invasion. In 308, probably April, Maximian attempted to depose his son in an assembly of soldiers in Rome. Surprisingly, present troops remained faithful to his son. Maximian had to flee to Constantine. At the conference of Carnuntum in autumn 308, Maxentius was again denied recognition as legitimate emperor. Licinius was appointed Augustus with the task of regaining the usurper's domain.
By middle of 310, Galerius had become too ill to involve himself in imperial politics. His final act survived as a letter posted in Nicomedia on the 30th of April 311 proclaiming end to persecutions of Christians. He died soon after the edict's proclamation. Maximinus mobilized against Licinius and seized Asia Minor. A hasty peace was signed on a boat in the middle of the Bosphorus. While Constantine toured Britain and Gaul, Maxentius prepared for war. He fortified northern Italy and strengthened Christian community support by allowing election of new Bishop of Rome, Eusebius. Maxentius' rule became insecure due to heightened tax rates and depressed trade. Riots broke out in Rome and Carthage. Domitius Alexander briefly usurped authority in Africa. By 312, he was barely tolerated even among Christian Italians. In summer 311, Maxentius mobilized against Constantine while Licinius was occupied in East. He declared war vowing to avenge his father's death. Constantine forged alliance with Licinius over winter 311-12 offering sister Constantia in marriage. Early spring 312 saw Constantine cross Cottian Alps with quarter of army numbering about 40,000. First town encountered was Segusium which shut gates. Constantine ordered men to set fire to gates and scale walls.
To east, on Galerius' death in 311 eastern provinces divided between Maximinus Daza and Licinius. Daza unhappy that Licinius made senior emperor took opportunity to declare himself emperor. Licinius held eastern European provinces while Daza took Asiatic provinces. In 312, while Constantine fought Maxentius, Daza campaigned against Armenians. He returned to Syria by February 313 discovering alliance forged by Constantine and Licinius in Mediolanum. Deciding to take initiative, Daza left Syria with 70,000 men reaching Bithynia though harsh weather weakened army. In April 313 he crossed Bosporus going to Byzantium held by Licinius troops. Undeterred, he took town after eleven-day siege. He moved to Heraclea captured after short siege before moving forces to first posting station. With much smaller body possibly around 30,000, Licinius arrived at Adrianople while Daza still besieging Heraclea. On the 30th of April 313 two armies clashed at Battle of Tzirallum where Daza's forces crushed. Ridding imperial purple and dressing like slave, Daza fled to Nicomedia. Believing chance to come out victorious remained, Daza attempted stop advance of Licinius at Cilician Gates establishing fortifications there. Unfortunately for Daza, Licinius' army succeeded breaking through forcing retreat to Tarsus where Licinius continued pressing on land and sea. War between them ended with Daza's death in August 313.
After Maxentius defeat Constantine gradually consolidated military superiority over rivals in crumbling Tetrarchy. In 313 he met Licinius in Milan securing alliance by marriage of Licinius and Constantia. Emperors agreed on Edict of Milan granting full tolerance to Christianity and all religions. Document had special benefits legalizing religion and restoring property seized during Diocletian persecution. Conference cut short when news reached Licinius that Maximinus crossed Bosporus invading European territory. Licinius departed eventually defeating Maximinus gaining control entire eastern half Roman Empire. Relations deteriorated though and in 316 hostilities prompted by Constantine appointment of brother-in-law Bassianus as Caesar. Bassianus discovered intriguing against Constantine perhaps prodded by own brother Senecio close associate of Licinius. When Constantine demanded handover Senecio, Licinius refused. Constantine marched against Licinius fighting war of Cibalae victorious. They clashed again Battle of Campus Ardiensis in 317 agreeing settlement making Crispus, Constantine II, and Licinianus caesars. In 320 Licinius reneged religious freedom promised by Edict of Milan beginning oppression Christians anew. It became challenge to Constantine west climaxing great civil war 324. Licinius aided Goth mercenaries represented past ancient Pagan faiths. Constantine and Franks marched under standard labarum both sides saw battle religious terms. Supposedly outnumbered but fired zeal Constantine's army emerged victorious Battle of Adrianople. Licinius fled across Bosphorus appointing Martius Martinianus commander bodyguard as Caesar. Constantine next won Battle of Hellespont finally Battle of Chrysopolis on the 18th of September 324. Licinius and Martinianus surrendered at Nicomedia promise lives spared sent live private citizens Thessalonica Cappadocia respectively. In 325 Constantine accused them plotting arrested hanged. Licinius son also eradicated thus Constantine sole emperor ending civil wars Tetrarchy.
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Common questions
When did Roman emperor Diocletian institute the Tetrarchy to end the Crisis of the Third Century?
Roman emperor Diocletian instituted the Tetrarchy in 293 AD. This administrative division split imperial power between two Augusti and two Caesars.
Who was elevated to Augustus by troops at Eburacum after the death of Constantius in 306 AD?
Troops at Eburacum elevated Constantine, son of Constantius, to the position of Augustus instead of accepting Severus. Galerius compromised by granting Constantine the title Caesar rather than Augustus while Severus received the latter office.
What happened to Maximinus Daza during the civil war that ended with his death in August 313?
Maximinus Daza mobilized against Licinius and seized Asia Minor before being defeated at the Battle of Tzirallum on the 30th of April 313. He fled to Nicomedia and attempted to stop the advance of Licinius at Cilician Gates but died in August 313.
Which battle marked the final victory for Constantine over Licinius on the 18th of September 324?
Constantine won the Battle of Chrysopolis on the 18th of September 324. This engagement forced Licinius and Martius Martinianus to surrender at Nicomedia and led to their eventual execution in 325.
When did the joint rule of Maxentius and Maximian face further testing when Galerius marched to Italy in summer 307?
Galerius marched to Italy in summer 307 with a larger army to challenge the joint rule of Maxentius and Maximian. During this invasion, Severus was put to death by Maxentius likely at Tres Tabernae while Galerius withdrew after soldiers defected from his invasion army.