350–353 Late period The siege of Byzantium was carried out some time between July and September 324 by the forces of the Roman emperor Constantine I (r. 306–337) during his Second Civil War against his rival, co-emperor Licinius (r. 308–324).

It would have been started simultaneously with the naval battle of the Hellespont (today known as Dardanelles) in which Constantine's son and caesar Crispus (r. 317–326) defeated the Lycinian navy commanded by Admiral Abanto.

However, he would be defeated at the consecutive battle of Chrysopolis, ending the Tetrarchy system and allowing the Constantine to establish himself as sole ruler of the Roman Empire.

Peace was quickly arranged after this, in which Constantine conquered all of the Balkan Peninsula, with the exception of Thrace,[1] and placed himself in a superior position to Licinius, leaving an unstable relationship between them.

As early as 323, Constantine was ready to renew the conflict, and when his army, which was chasing an invading band of Visigoths (or Sarmatians), crossed the border into Licinius' territory, a timely casus belli was present.