On July 3, 324, at the Battle of Adrianople, Roman Emperor Constantine I defeated his co-emperor Licinius, who fled to Byzantium. An excerpt from the article:
"The underlying causes of the battle were -again- the rivalries that started after the collapse of Diocletian's Tetrarchy. Constantine had prevailed in the West after the battle in the Milvian bridge (312) and didn’t avoid a conflict with the emperor of the East (in 316), when he defeated Licinius and conquered all the Balkan Peninsula, except Thrace. A peace had been arranged that lasted 7 yeras.
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, a convenient casus belli was created. Licinius overacted, his reaction was taken as hostile and the civil war started again. Constantine invaded Thrace with a large army."