The Danubian Provinces of Rome. Adrianople (Hadrianoplis) and the Hebrus River are shown in the Province of Thrace
Constantine had, in a previous war (in 316), defeated Licinius at the Battle of Cibalae and conquered from him all the Balkan Peninsula, with the exception of 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.