Battle of Adrianople ( 324 ) The Battle of Adrianople was fought on July 3 , 324 during a Roman civil war , the second to be waged between the two emperors Constantine I and Licinius ; Licinius suffered a heavy defeat . Background . Constantine had , in a previous war ( in 316 ) , defeated Licinius at the Battle of Cibalae and conquered from him all the Balkan Peninsula , excepting Thrace . A peace had been arranged but the relationship between the two emperors remained uneasy . By 324 Constantine was ready to renew the conflict and when his army , in pursuit of a raiding Visigothic , or possibly Sarmatian , force , crossed into Licinius ' territory an opportune " casus belli " was created . The reaction of Licinius to this incursion was overtly hostile and this induced Constantine to go on to the offensive . Constantine invaded Thrace in force ; his army was smaller than that of Licinius , but it contained many battle - hardened veterans and , as he had control of the Illyrian region , the finest quality of new recruits . Battle . Licinius encamped his army at Adrianople ( Hadrianopolis ) , the major city of inland Thrace . Constantine advanced eastward from Thessalonica until he came to the Hebrus River , on which Adrianople stands , and set up his own camp . Licinius arranged his battle line , of 200 stade s in length , in a strong position between a height overlooking the town and the confluence of the Hebrus with a tributary . The two armies remained in position for a number of days before battle was joined , as both sides were reluctant to chance the crossing of the river against a well - prepared and battle - arrayed enemy . Eventually , Constantine used a ruse to get his troops across the Hebrus . Having noticed a suitable crossing point where the river narrowed and was overlooked by a wooded hillside , he ordered material and ropes to be conspicuously assembled at another place on the river , well away from his chosen crossing , to give the impression that he intended to build a bridge to cross there . On the wooded hillside he secretly assembled 5 , 000 foot archers and a force of cavalry . He then led his cavalry over the river crossing at the narrows and fell on the enemy unexpectedly . The surprise attack was a complete success and the remainder of his army then crossed at the same point . What followed , in the words of the historian Zosimus , was " a great massacre " : Licinius ' army lost about 34 , 000 dead . During the onslaught Constantine directed the guard of his overtly Christian standard , the labarum , to move it to any part of the field where his troops seemed to be faltering . The appearance of this talisman emboldened his own troops and dismayed those of Licinius . Constantine , who had been slightly wounded in the thigh , halted his attack at sunset and darkness allowed Licinius and the remains of his force to withdraw to Byzantium , the coast , and the safety of his fleet . The battle was one of the largest of the 4th century . Aftermath . Constantine ' s effort to start a civil conflict proved successful , as did his campaign against Licinius . Following the battle at Adrianople Constantine besieged Byzantium , whilst his son Crispus commanded his navy in a struggle to wrest command of the narrow waters separating Thrace and Asia Minor from Licinius ' fleet . Following Crispus ' naval victory , in the waters of the Hellespont , Constantine crossed with his army into Bithynia . He met Licinius ' army in the final battle of the war at Chrysopolis on the Asiatic shore of the Bosporus . Constantine won an overwhelming victory . Initially , yielding to the pleas of his sister Constantia , 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 . Constantine was then the sole and undisputed master of the Roman world . References . Primary source Secondary sources