Civil wars of the Tetrarchy The Civil wars of the Tetrarchy were a series of conflicts between the co - emperors of the Roman Empire , starting in 306 AD with the usurpation of Maxentius and the defeat of Severus , and ending with the defeat of Licinius at the hands of Constantine I in 324 AD . Background . The Tetrarchy refers to the administrative division of the Roman Empire instituted by Roman Emperor Diocletian in 293 AD , marking the end of the Crisis of the Third Century and the recovery of the Roman Empire . The first phase , sometimes referred to as the Diarchy ( ' the rule of two ' ) , involved the designation of the general Maximian as co - emperor - firstly as " Caesar " ( junior emperor ) in 285 , followed by his promotion to " Augustus " in 286 . Diocletian took care of matters in the Eastern regions of the Empire while Maximian similarly took charge of the Western regions . 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 " . They in turn appointed two new Caesars - Severus in the west under Constantius , and Maximinus Daia in the east under Galerius . Opening gambits . The death of Constantius at York in 306 AD saw the first crack in the political edifice of the Tetrarchy . Rather than accepting the elevation of Severus from Caesar to Emperor , the troops at York elevated Constantius ’ son , Constantine , to the position of emperor . Contacting Galerius as the senior emperor , he requested recognition as heir to his father ' s throne , and passed off responsibility for his unlawful ascension on his army , claiming they had " forced it upon him " . Galerius was put into a fury by the message ; he almost set the portrait on fire . His advisers calmed him , and argued that outright denial of Constantine ' s claims would mean certain war . Galerius was compelled to compromise : he granted Constantine the title " Caesar " rather than " Augustus " ( the latter office went to Severus instead ) . Wishing to make it clear that he alone gave Constantine legitimacy , Galerius personally sent Constantine the emperor ' s traditional purple robes . Constantine accepted the decision , knowing that it would remove doubts as to his legitimacy . This act motivated Maxentius , the son of Maximian to also declare himself Emperor at Rome in 306 AD . Galerius , by now fearful that others would also attempt to become emperor , ordered Severus into Italy to deal with the usurper . Severus moved from his capital , Mediolanum , towards Rome , at the head of an army previously commanded by Maximian . Fearing the arrival of Severus , Maxentius offered Maximian the co - rule of the empire . Maximian accepted , and when Severus arrived under the walls of Rome and besieged it , his men deserted him and passed to Maximian , their old commander . Severus fled to Ravenna , an impregnable position : Maximian offered to spare his life and treat him humanely if the latter surrendered peaceably , which he did in March or April 307 . Despite Maximian ' s assurance , Severus was nonetheless displayed as a captive and later imprisoned at Tres Tabernae , before being eventually killed . 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 . While negotiating with the invader , Maxentius could repeat what he did to Severus : by the promise of large sums of money , and the authority of Maximian , many soldiers of Galerius defected to him . Galerius was forced to withdraw , plundering Italy on his way . Some time during the invasion , Severus was put to death by Maxentius , probably at Tres Tabernae near Rome ( the exact circumstances of his death are not certain ) . After the failed campaign of Galerius , Maxentius ' reign over Italy and Africa was firmly established . Beginning in 307 already , he tried to arrange friendly contacts with Constantine , and in the summer of that year , Maximian travelled to Gaul , where Constantine married his daughter Fausta and was in turn appointed Augustus by the senior emperor . However , Constantine tried to avoid breaking with Galerius , and did not openly support Maxentius during the invasion . In 308 , probably April , Maximian tried to depose his son in an assembly of soldiers in Rome ; surprisingly to him , the present troops remained faithful to his son , and he had to flee to Constantine . 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 . In 310 , Maximian rebelled against Constantine while the emperor was on campaign against the Franks . Maximian had been sent south to Arles with part of Constantine ' s army to defend against attacks by Maxentius in southern Gaul . In Arles , Maximian announced that Constantine was dead and took up the imperial purple . Despite offering bribes to any who would support him as emperor , 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 ( Marseille ) . 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 . Maximian was captured , reproved for his crimes , and stripped of his title for the third and last time . Constantine granted Maximian some clemency but strongly encouraged his suicide . In July 310 , Maximian hanged himself . War of Constantine and Maxentius . By the middle of 310 Galerius had become too ill to involve himself in imperial politics . 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 . He died soon after the edict ' s proclamation , destroying what little stability remained in the tetrarchy . 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 his support in the Christian community by allowing it to elect a new Bishop of Rome , Eusebius . Maxentius ' rule was nevertheless insecure . His early support dissolved in the wake of heightened tax rates and depressed trade ; riots broke out in Rome and Carthage ; and Domitius Alexander was able to briefly usurp his authority in Africa . By 312 , he was a man barely tolerated , not one actively supported , even among Christian Italians . In the summer of 311 , Maxentius mobilized against Constantine while Licinius was occupied with affairs in the East . He declared war on Constantine , vowing to avenge his father ' s " murder " . To prevent Maxentius from forming an alliance against him with Licinius , Constantine forged his own alliance with Licinius over the winter of 311 – 12 , and offered him his sister Constantia in marriage . Maximinus considered Constantine ' s arrangement with Licinius an affront to his authority . In response , he sent ambassadors to Rome , offering political recognition to Maxentius in exchange for a military support . Maxentius accepted . According to Eusebius , inter - regional travel became impossible , and there was military buildup everywhere . There was " not a place where people were not expecting the onset of hostilities every day " . Constantine ' s advisers and generals cautioned against preemptive attack on Maxentius ; even his soothsayers recommended against it , stating that the sacrifices had produced unfavorable omens . Constantine , with a spirit that left a deep impression on his followers , inspiring some to believe that he had some form of supernatural guidance , ignored all these cautions . Early in the spring of 312 , Constantine crossed the Cottian Alps with a quarter of his army , a force numbering about 40 , 000 . The first town his army encountered was Segusium ( Susa , Italy ) , a heavily fortified town that shut its gates to him . Constantine ordered his men to set fire to its gates and scale its walls . He took the town quickly . Constantine ordered his troops not to loot the town , and advanced with them into northern Italy . 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 . In the ensuing battle Constantine ' s army encircled Maxentius ' cavalry , flanked them with his own cavalry , and dismounted them with blows from his soldiers ' iron - tipped clubs . Constantine ' s armies emerged victorious . Turin refused to give refuge to Maxentius ' retreating forces , opening its gates to Constantine instead . Other cities of the north Italian plain sent Constantine embassies of congratulation for his victory . He moved on to Milan , where he was met with open gates and jubilant rejoicing . Constantine rested his army in Milan until mid - summer 312 , when he moved on to Brixia ( Brescia ) . Brescia ' s army was easily dispersed , and Constantine quickly advanced to Verona , where a large Maxentian force was camped . Ruricius Pompeianus , general of the Veronese forces and Maxentius ' praetorian prefect , was in a strong defensive position , since the town was surrounded on three sides by the Adige . Constantine sent a small force north of the town in an attempt to cross the river unnoticed . Ruricius sent a large detachment to counter Constantine ' s expeditionary force , but was defeated . Constantine ' s forces successfully surrounded the town and laid siege . Ruricius gave Constantine the slip and returned with a larger force to oppose Constantine . Constantine refused to let up on the siege , and sent only a small force to oppose him . In the desperately fought encounter that followed , Ruricius was killed and his army destroyed . Verona surrendered soon afterwards , followed by Aquileia , Mutina ( Modena ) , and Ravenna . The road to Rome was now wide open to Constantine . Maxentius prepared for the same type of war he had waged against Severus and Galerius : he sat in Rome and prepared for a siege . 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 , and left the rest of central Italy undefended ; Constantine secured that region ' s support without challenge . Constantine progressed slowly along the " Via Flaminia " , allowing the weakness of Maxentius to draw his regime further into turmoil . Maxentius ' support continued to weaken : at chariot races on 27 October , the crowd openly taunted Maxentius , shouting that Constantine was invincible . 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 . On 28 October 312 , the sixth anniversary of his reign , he approached the keepers of the Sibylline Books for guidance . The keepers prophesied that , on that very day , " the enemy of the Romans " would die . Maxentius advanced north to meet Constantine in battle . Maxentius organized his forces — still twice the size of Constantine ' s — in long lines facing the battle plain , with their backs to the river . Constantine ' s army arrived at the field bearing unfamiliar symbols on either its standards or its soldiers ' shields . According to Lactantius , Constantine was visited by a dream the night before the battle , wherein he was advised " to mark the heavenly sign of God on the shields of his soldiers . . . by means of a slanted letter X with the top of its head bent round , he marked Christ on their shields . " 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 " ; 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 . Eusebius is vague about when and where these events took place , but it enters his narrative before the war against Maxentius begins . 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 . The Eusebian description of the vision has been explained as a " solar halo " , a meteorological phenomenon which can produce similar effects . In 315 a medallion was issued at Ticinum showing Constantine wearing a helmet emblazoned with the Chi - Rho , and coins issued at Siscia in 317 / 18 repeat the image . The figure was otherwise rare , however , and is uncommon in imperial iconography and propaganda before the 320s . Constantine deployed his own forces along the whole length of Maxentius ' line . He ordered his cavalry to charge , and they broke Maxentius ' cavalry . He then sent his infantry against Maxentius ' infantry , pushing many into the Tiber where they were slaughtered and drowned . The battle was brief : Maxentius ' troops were broken before the first charge . Maxentius ' horse guards and praetorians initially held their position , but broke under the force of a Constantinian cavalry charge ; they also broke ranks and fled to the river . 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 . Constantine entered Rome on 29 October . He staged a grand " adventus " in the city , and was met with popular jubilation . Maxentius ' body was fished out of the Tiber and decapitated . His head was paraded through the streets for all to see . After the ceremonies , Maxentius ' disembodied head was sent to Carthage ; at this Carthage would offer no further resistance . War of Licinius and Maximinus Daia . Meanwhile to the east , on Galerius ’ death in 311 the eastern provinces were divided between Maximinus Daia and Licinius . Daia was unhappy that Licinius had been made senior emperor by Galerius , and took the first available opportunity to declare himself as emperor . Licinius held the eastern European provinces , while Daia took possession of the Asiatic provinces . In the autumn of 312 , while Constantine was fighting Maxentius , Daia was busy campaigning against the Armenians . He had returned to Syria by February 313 when discovered the alliance that had been forged by Constantine and Licinius . He in turn entered into a secret alliance with Maxentius , but was soon flustered by the death of Maxentius . Deciding to take the initiative , Daia left Syria with 70 , 000 men and reached Bythinia , although harsh weather he encountered along the way had gravely weakened his army . In April 313 , he crossed the Bosporus and went to Byzantium , which was held by Licinius ' troops . Undeterred , he took the town after an eleven - day siege . He moved to Heraclea , which he captured after a short siege , before moving his forces to the first posting station . With a much smaller body of men , possibly around 30 , 000 , Licinius arrived at Adrianople while Daia was still besieging Heraclea . On 30 April 313 , the two armies clashed at the Battle of Tzirallum , and in the ensuing battle Daia ' s forces were crushed . Ridding himself of the imperial purple and dressing like a slave , Daia fled to Nicomedia . Believing he still had a chance to come out victorious , Daia attempted to stop the advance of Licinius at the Cilician Gates by establishing fortifications there . Unfortuately for Daia , Licinius ' army succeeded in breaking through , forcing Daia to retreat to Tarsus where Licinius continued to press him on land and sea . The war between them only ended with Daia ’ s death in August 313 . Wars of Constantine and Licinius . After Maxentius ’ defeat , Constantine gradually consolidated his military superiority over his rivals in the crumbling Tetrarchy . In 313 , he met Licinius in Milan to secure their alliance by the marriage of Licinius and Constantine ' s half - sister Constantia . During this meeting , the emperors agreed on the so - called Edict of Milan , officially granting full tolerance to " Christianity and all " religions in the Empire . The document had special benefits for Christians , legalizing their religion and granting them restoration for all property seized during Diocletian ' s persecution . It repudiates past methods of religious coercion and used only general terms to refer to the divine sphere — " Divinity " and " Supreme Divinity " , " summa divinitas " . The conference was cut short , however , when news reached Licinius that his rival Maximin 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 . Relations between the two remaining emperors deteriorated , though , and either in 314 or 316 , Constantine and Licinius fought against one another in the war of Cibalae , with Constantine being victorious . They clashed again in the Battle of Campus Ardiensis in 317 , and agreed to a settlement in which Constantine ' s sons Crispus and Constantine II , and Licinius ' son Licinianus were made " caesars " . In the year 320 , Licinius reneged on the religious freedom promised by the Edict of Milan in 313 and began to oppress Christians anew . It became a challenge to Constantine in the west , climaxing in the great civil war of 324 . Licinius , aided by Goth mercenaries , represented the past and the ancient Pagan faiths . Constantine and his Franks marched under the standard of the " labarum " , and both sides saw the battle in religious terms . Supposedly outnumbered , but fired by their zeal , Constantine ' s army emerged victorious in the Battle of Adrianople . Licinius fled across the Bosphorus and appointed Martius Martinianus , the commander of his bodyguard , as Caesar , but Constantine next won the Battle of the Hellespont , and finally the Battle of Chrysopolis on 18 September 324 . Licinius and Martinianus surrendered to Constantine at Nicomedia on the promise their lives would be spared : they were sent to live as private citizens in Thessalonica and Cappadocia respectively , but in 325 Constantine accused Licinius of plotting against him and had them both arrested and hanged ; Licinius ' s son ( the son of Constantine ' s half - sister ) was also eradicated . Thus Constantine became the sole emperor of the Roman Empire , ending the Civil Wars of the Tetrarchy .