Battle of Chrysopolis The Battle of Chrysopolis was fought on 18 September 324 at Chrysopolis ( Üsküdar ) , near Chalcedon ( Kadıköy ) , between the two Roman emperors Constantine I and Licinius . The battle was the final encounter between the two emperors . After his navy ' s defeat in the Battle of the Hellespont , Licinius withdrew his forces from the city of Byzantium across the Bosporus to Chalcedon in Bithynia . Constantine followed , and won the subsequent battle . This left Constantine as the sole emperor , ending the period of the Tetrarchy . Background . In the Battle of the Hellespont Licinius ' navy had suffered a catastrophic defeat . His admiral , Abantus , had been outfought by Constantine ' s son the caesar Crispus , despite the latter ' s distinctly smaller fleet . Following this naval victory , Constantine crossed over to Asia Minor . He used a flotilla of light transports he had ordered to be built on the Bosporus in order to avoid the enemy army , which , under the command of Licinius ' newly appointed co - emperor Martinian , was guarding the coast at Lampsacus on the Hellespont . Following the destruction of his naval forces Licinius evacuated his garrison from Byzantium which joined his main army in Chalcedon on the Asiatic shore of the Bosporus . From there he also summoned Martinian ' s forces and a band of Visigothic auxiliaries , under their leader Aliquaca ( or Alica ) , to reinforce his principal army which had been depleted by its earlier defeat at the Battle of Adrianople . It is not clear whether Martinian ' s forces reached Licinius before September 18 when Licinius was brought to battle by Constantine . Battle . Constantine ' s army landed on the Asiatic shore of the Bosphoros at a place called the Sacred Promontory and marched southward towards Chalcedon . Licinius moved his army a few miles north towards Chysopolis . Constantine ' s army reached the environs of Chrysopolis before the forces of Licinius . Following a retreat to his tent to seek divine guidance , Constantine decided to take the initiative . The religious aspect of the conflict was reflected in Licinius drawing up his battle lines with images of the pagan gods of Rome prominently displayed , whilst Constantine ' s army fought under his talismanic Christian standard , the labarum . Licinius had developed a superstitious dread of the labarum and forbade his troops from attacking it , or even looking directly at it . Constantine seemingly eschewed any subtlety of manoeuvre , he launched a single massive frontal assault on Licinius ' troops and routed them . He won a decisive victory in what was a very large - scale battle . According to the historian Zosimus , " There was great slaughter at Chrysopolis . " Licinius was reported to have lost 25 , 000 to 30 , 000 dead , with thousands more breaking and running in flight . Licinius managed to escape and gathered around 30 , 000 of his surviving troops at the city of Nicomedia . Aftermath . Licinius , recognising that his surviving forces in Nicomedia could not stand against Constantine ' s victorious army , was persuaded to throw himself on the mercy of his enemy . Constantia , Constantine ' s half - sister and Licinius ' wife , acted as intermediary . Initially , yielding to the pleas of his sister , Constantine spared the life of his brother - in - law , but some months later he ordered his execution , thereby breaking his solemn oath . Licinius was suspected of treasonable actions and the army command pressed for his execution . A year later , Constantine ' s nephew the younger Licinius also fell victim to the emperor ' s anger or suspicions . In defeating his last foe , Licinius , Constantine became the sole emperor of the Roman empire ; the first since the elevation of Maximian to the status of augustus by Diocletian in April 286 . After his conquest of the eastern portion of the Roman Empire Constantine made the momentous decision to give the east its own capital , and the empire as a whole its second . He chose the city of Byzantium — renamed " Constantinopolis " — as the site of this new foundation . References . Primary sources Secondary sources