The Roman system of the Tetrarchy — four co-emperors sharing rule of a vast empire — had been unraveling for years. By 324, only two were left: Constantine, ruling the western half, and Licinius, ruling the east. They had already fought once, eight years earlier at the battles of Cibalae and Campus Mardiensis. Peace had followed, uneasy and transactional, with Constantine claiming the Balkans and Licinius retreating behind an unstable arrangement.

The casus belli for their second war arrived in 323, when Constantine's army, chasing Visigoths or Sarmatians who had crossed the border, entered Licinius's territory. Licinius chose to treat the incursion as a declaration of war. He was probably not wrong about Constantine's intentions — the man had never stopped positioning himself for dominance. Constantine invaded Thrace with veteran forces. At the Battle of Adrianople in 324, Licinius suffered a decisive defeat and retreated with his army to the walls of Byzantium.

Licinius's position after Adrianople was strategically coherent but increasingly precarious. Byzantium's walls were strong, and he left a garrison inside the city while crossing the Bosphorus himself with the bulk of his forces, hoping to regroup in Asia Minor. The logic required controlling the water: if his navy could hold the Hellespont and the Bosphorus, Constantine could not follow him into Asia.

Constantine understood this. He began the land siege of Byzantium — building embankments as high as the city walls, deploying battering rams, erecting wooden siege towers from which archers could fire down on the defenders — while simultaneously directing a naval campaign at the Hellespont. The naval battle was commanded by his son Crispus, then holding the rank of caesar. Crispus defeated the Licinian fleet under Admiral Abantus at the Hellespont, breaking Licinius's naval screen. With the sea route open, supplies flowed freely to Constantine's besieging army. The outcome of the siege was no longer in doubt.

Licinius, reading the situation clearly, did not wait for the walls to be breached. He abandoned Byzantium and the weakened garrison he had left inside, withdrew to Chalcedon in Bithynia across the Bosphorus, and prepared to make a final stand. Constantine moved the bulk of his troops to Anatolia and met him at the Battle of Chrysopolis — a battle Licinius lost decisively. Byzantium and Chalcedon surrendered. Licinius fled to Nicomedia and eventually submitted.

The soldiers who died in the siege of Byzantium — its defenders cut down or drowned in the Bosphorus attempting to escape, the attackers who fell to arrows or sorties from the walls — died in what was formally a Roman civil war, one soldier of the empire against another. Constantine accepted Licinius's surrender and had him executed shortly afterward, ending the Tetrarchy permanently. The sole ruler of the Roman Empire now stood in a city whose potential he recognized at once.

Constantine did not linger long in contemplation of Byzantium's ruins and blood. He began planning what would become Constantinople — the city he would formally dedicate in 330 AD as Nova Roma, the New Rome. The Hippodrome was expanded; monuments were transported from across the empire; a new imperial palace complex rose behind the old Greek acropolis. The city that Constantine had just spent months attacking with battering rams and siege towers became the center of a civilization that would endure for more than a thousand years.

The siege of 324 is remembered now primarily as a stepping stone — the battle that cleared the path to sole rule, and thus to the founding of Constantinople. But it was also, in the narrow sense, a genuine military operation: an army crossing a contested strait, towers built against ancient walls, men dying on both sides of a dispute about who would hold an empire neither of them had built. The city they fought over would outlast them both by eleven centuries.