 This dubious arrangement eventually 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 . Outnumbered , but fired by their zeal , Constantine 's army emerged victorious in the Battle of Adrianople . Licinius fled across the Bosphorus and appointed Martius <unk> , 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 <unk> 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 killed . Thus Constantine became the sole emperor of the Roman Empire . 
