Battle of Chrysopolis, 324: Constantine becomes sole emperor

Constantine and Licinius were the last co-emperors of the Tetrarchy, the system of shared rule created by Diocletian. Constantine held the West, Licinius the East, and the alliance between them was sealed when Constantine gave his sister Constantia to Licinius in marriage at Milan. It did not last. Constantine wanted to rule a united empire, and his administrative moves soon set the two at odds.²

Their first war was decided at Cibalae, in modern Croatia, and on the Campus Ardiensis in Thrace. Its dating is one of the great puzzles of the period: Michael DiMaio notes that scholars place it in 314 or in 316 and that the literary, coin and legal evidence will probably never settle the question. Licinius kept his throne but had to give up much of the Balkans, and he moved his capital to Nicomedia.³

Religion and the road to war

Religion sharpened the quarrel. Although the two emperors had issued the Edict of Milan together in 313, Licinius turned against the Christians of his realm around 320. He forbade bishops to correspond with each other or meet in synods, and dismissed army officers who would not sacrifice to the gods. In Turhan Kaçar's reading, Licinius banned the meetings of bishops to prevent disorder in the cities during the bitter debates on the nature of Christ, and Constantine seized the chance to present himself as the defender of the Christians, comparing Licinius in a speech at Serdica to the persecutors of the Church.⁴

The pretext came on the Danube. In 323 barbarian raiders broke into Thrace and Moesia, and Constantine, pursuing them, crossed several times into Licinius' territory. Licinius answered by refusing to let coins minted in Constantine's name circulate in the East.⁵

Adrianople and the Hellespont

In the summer of 324 Constantine assembled a fleet and an army at Thessalonica and marched on Adrianople. On 3 July he crossed the river Hebrus and routed Licinius, who retreated by night to Byzantium; the next day a large part of his army surrendered. Licinius then put his hopes in his navy, which guarded the Hellespont. His admiral, Abantus, was beaten by bad weather and by a smaller fleet under Constantine's eldest son, Crispus.⁶

Besieged in Byzantium by land and sea, Licinius abandoned the city and crossed with most of his army and treasure to Chalcedon on the Asian shore. He left Martinianus, his former chief of the palace offices, whom he had raised to co-emperor, to hold up Constantine's advance.⁷

The battle on the Asian shore

Constantine did not try to force the narrow crossing under the enemy's eyes. According to Kaçar, he took his army over the Bosphorus near its mouth on the Black Sea, at a sacred place belonging to the Chalcedonians some two hundred stadia, about 35 kilometers, north of Chalcedon. From there he advanced through the hills around present-day Beykoz and marched south, while Licinius, seeing Bithynia threatened, came north to meet him.⁸

The armies met on 18 September 324 at Chrysopolis, between Chalcedon and the place where Constantine had landed; the town is today Üsküdar, a district of Istanbul. The ancient narratives say little about the fighting itself, but they agree that it was fierce and decisive. According to the sources cited by Kaçar, about 25,000 of Licinius' troops were killed. Licinius fled to Nicomedia.⁹

The end of Licinius

Chalcedon opened its gates to Constantine, and Byzantium followed; Kaçar recalls that the city had paid dearly 130 years earlier for backing the loser in the war between Septimius Severus and Pescennius Niger. Constantine besieged Nicomedia, and on the next day Licinius abdicated. He was sent to Thessalonica under house arrest. Martinianus was probably executed before the end of 324, and Licinius in the spring of 325, amid rumors that he had plotted a new rebellion.¹⁰

From Byzantium to Constantinople

One man now ruled the whole Roman world, from the Danube to the Euphrates. Constantine saw quickly that such an empire could be governed best from a place like Byzantium, on the crossing between Europe and Asia. Later writers such as Sozomen and Zosimus claim that he first began building a capital at Troy, and a medieval tale has an eagle carry a stone from the walls he was raising at Chalcedon over to Byzantium. Kaçar trusts instead the anonymous Origo Constantini, written a few years after the emperor's death, which says that Constantine named Byzantium Constantinople in memory of his great victory, and dates the decision to rebuild the city to 1 November 324.¹¹

The new city was meant to rival Rome. Constantine stripped Aegean cities of their monuments to adorn it, invited rich families from Asia Minor to settle there and created a senate of his own. As Kaçar observes, had Licinius won he would have had no need for a new eastern capital, since he already had Nicomedia; the founding of Constantinople, the capital of an empire for more than a thousand years, was a direct result of the battle at Chrysopolis.¹²