Battle of Chrysopolis

18 September 324, in Bithynia

Constantine landed troops in Bithynia and met Licinius's army between Chalcedon and the Sacred Promontory on 18 September 324. Licinius had already been beaten near Adrianople and at sea in the Hellespont; the landing forced him to fight before his position could recover.

Constantine's army won decisively. The Anonymus Valesianus reports 25,000 Licinian soldiers killed and the remainder put to flight — a claim in a later transmitted narrative, not a modern casualty count.

The famous numbers are two incompatible numbers

Zosimus, writing in the early sixth century, claims Licinius fielded 130,000 men and that scarcely 30,000 escaped. The often-repeated "25,000 to 30,000 dead" fuses those escapees with the Anonymus's death figure.

Zosimus also says Licinius reached Nicomedia with his remaining cavalry and only a few thousand infantry — not with 30,000 survivors regrouping there.

The sources give little secure tactical detail; the stories of opposed pagan standards and Licinius's fear of the labarum come from strongly partisan Constantinian Christian tradition.

Surrender, clemency, and a delay

When news of the defeat arrived, Byzantium and Chalcedon opened their gates to Constantine.

The next day Constantia — Constantine's half-sister and Licinius's wife — entered the camp and obtained clemency for her husband. Licinius relinquished imperial rank, and his recently appointed co-ruler Martinianus was initially spared as well.

The surrender left Constantine the empire's sole reigning Augustus. He had Licinius killed later, probably in the spring of 325.