In the spring of 324 Constantine and Licinius were both ready for war. According to Zosimus, Constantine had gathered an army of almost 120,000 infantry and 10,000 cavalry, while Licinius commanded almost 150,000 infantry and 15,000 cavalry. These numbers may remind us of the battle of Lugdunum, fought in the year 197 between Septimius Severus and Clodius Albinus. The numbers are no doubt inflated, as they obviously were in the case of the previous battle. A total of almost 300,000 soldiers would, after all, constitute three quarters of the whole Roman army. If so many soldiers had really been removed from the provinces and borders, these would have been left seriously weakened and vulnerable to attack. Nevertheless, both armies must have been large, with Constantine perhaps commanding about 40,000 men and Licinius some 50,000. Moreover, Constantine had gathered a transport fleet in Thessalonica consisting of several hundred ships, which could follow the army along the coast as it marched through Macedonia and Thrace. He had furthermore gathered a fleet of 200 warships in Piraeus near Athens. This fleet was commanded by his eldest son Crispus. Licinius in his turn had sent a fleet of 350 warships under a certain Abantus or Amandus to the Hellespont.
Battle of Hadrianopolis
From Thessalonica Constantine marched along the Via Egnatia to Hadrianopolis in Thrace. Thrace and Macedonia were separated from each other by the river Hebrus, and Hadrianopolis was situated on the other side of the water. Constantine had to find a way to cross the river, and in the meantime he ordered his army to make camp. On the other side of the Hebrus Licinius had deployed his army in battle order on the high ground, with one of his flanks protected by a tributary of the Hebrus. And so the two armies started facing each other for a number of days. The upcoming battle was certainly not a clash between a Christian army (that of Constantine, who may or may not have considered himself a Christian at the time) and a “pagan” army (that of Licinius, who still put his trust in Jupiter). Constantine’s army must have numbered several hundred or perhaps a few thousand Christian soldiers, in addition to the fifty elite soldiers that guarded the labarum, Constantine’s battle standard with the chi-rho sign. But the vast majority of the men who would fight at Hadrianopolis, also in Constantine’s army, still followed the traditional cults.
Zosimus suggests that Licinius’ battle line was 200 stadia wide, which would amount to over 30 kilometres and can therefore hardly be considered realistic. If this is not a copyist’s mistake, then perhaps the historian meant that Licinius had spread out his forces across a wide area because there were several spots where Constantine might cross the Hebrus. The latter came up with a clever ruse to solve the problem of the river. He pretended to make preparations to build a bridge in one spot by cutting down trees, tying ropes around the logs and having these taken to the river. Meanwhile, however, he had found a place where the river was narrow and easily fordable. Here, on 3 July, the emperor ordered a small vanguard to cross the Hebrus. It quickly drove away the enemy troops it encountered, encouraging the rest of Constantine’s army to make the crossing as well. Now the battle of Hadrianopolis started in earnest.
Although he had managed to get across the water, Constantine had not won the battle yet. Licinius still occupied the high ground, and Constantine’s troops had to fight hard to expel them from there. The Origo Constantini Imperatoris even claims that Constantine himself was wounded in the thigh during the battle. This may very well be later propaganda, as the emperor was well into his fifties and is unlikely to have fought on horseback in the front ranks (a thigh wound is typical for a horseman). Both sides must have suffered substantial losses, with Zosimus claiming there were 30,000 dead without mentioning on whose side they fought. Around sunset the battle was over. Constantine took Licinius’ camp, and the latter fled to Byzantium. Many of his soldiers that had stayed behind then surrendered to Constantine’s army. Constantine continued to follow the Via Egnatia and subsequently laid siege to Byzantium. The city seemed like an impenetrable fortress, well protected on both the land and sea sides. As long as Abantus still controlled the access to the Propontis (the Sea of Marmara), Licinius had little to fear.
Battles of the Hellespont and Chrysopolis
Now it was time for Crispus and his fleet to act. At Callipolis (Gallipoli) his vanguard of about 80 ships defeated a much larger but badly led fleet of 200 ships under Abantus. Nightfall ended the fighting, and Crispus and Abantus took their fleets to anchoring spots. Crispus had chosen his spot better than his opponent, as the largest part of Abantus’ fleet was subsequently destroyed by a storm. Licinius’ admiral supposedly then fled to the province of Asia with a mere four ships, which allowed Crispus to sail through the Bosporus towards Byzantium. At the city, Constantine’s siege had made considerable progress. The emperor had ordered the construction of a ramp (agger) and siege towers (turres), and had his men pepper the enemy from the towers with a hail of missiles. Battering rams were furthermore prepared to breach the walls.
Licinius realised he was in grave danger and quickly decided to leave the city with his best troops before Crispus would succeed in cutting Byzantium off on the sea side. He crossed over to Chalcedon, where he appointed his magister officiorum Martinianus augustus. While Licinius was busy entrenching himself in and around Chalcedon, he sent his co-emperor to Lampsacus (opposite Callipolis) to stop the enemy if he attempted to cross over from Thrace to Asia Minor. But Lampsacus proved to be the wrong spot, as Constantine managed to cross the Bosporus at the “sacred cape”, a few dozen kilometres north of Chalcedon. The move took Licinius completely by surprise. On 18 September the final confrontation between the two brothers-in-law took place at Chalcedon. Licinius made use of Gothic auxiliaries, but suffered a crushing defeat. However, Zosimus’ claim that of his 130,000 men only 30,000 were still alive after the battle must be taken with a pinch of salt. With the remainder of his army Licinius now fled to Nicomedia, the capital of Bithynia. Not long after the battle of Chrysopolis both Byzantium and Chalcedon surrendered to Constantine.
There would be no more fighting at Nicomedia. Through the mediation of Constantia, Constantine’s half-sister and Licinius’ wife, the latter surrendered to his brother-in-law. In a solemn ceremony, Licinius took off his purple robes, recognised Constantine as the sole legitimate emperor and asked him for forgiveness. Martinianus was executed soon after the surrender, but Constantine had given Constantia his word that her husband would be spared. It was a promise he would keep for just a couple of months. Licinius was sent to Thessalonica, where Constantine had him murdered in the spring of 325. The precise motive for the murder has remained unclear. Perhaps Constantine’s soldiers demanded the execution of their former enemy, perhaps Licinius was hatching plots from his new place of residence, but it is also possible that Constantine simply wanted to get rid of his rival. Every other Roman emperor would probably have done the same. Not much later, possibly in 326, Licinius’ son Licinius junior was also killed. Constantine was the sole emperor of the vast Roman Empire and potential rivals had to be dealt with.
Government and religion
Now that the Empire had been united again under a single emperor, Constantine took a couple of important steps. First of all, on 8 November 324 he made his second son Constantius II, now about seven years old, a caesar. This meant that the Roman Empire now had one augustus – Constantine himself obviously – and three caesares, the princes Crispus, Constantine II and Constantius II. On the same day the emperor consecrated a large terrain near the city of Byzantium to found the new city of Constantinople. Large-scale building activities were subsequently launched. The new city’s location, on the border between Europe and Asia Minor, was of major strategic importance. In 196 the emperor Septimius Severus had pillaged and destroyed Byzantium, but the city had been rebuilt soon after, precisely because of that strategic importance. From the start, it was crystal clear that Constantine wanted to make the new city, which covered a much larger surface than ancient Byzantium, his capital, a New Rome. Ironically, the day after the consecration of the terrain the large Basilica of the Saviour in Old Rome, sponsored by Constantine himself, was inaugurated, at least according to tradition. But Constantine was more or less done with Old Rome. His future lay further to the east.
The Roman east, however, was torn to pieces by religious strife. The young Christian religion was faced with two major points of contention: the date for Easter and the nature of Christ. As regards this nature, the priest Arius of Alexandria had stated that Jesus was not of the same substance as God, but had been created by the latter as a human being. A perfect human being, certainly, but a human being nonetheless, and therefore subordinate to the Father. This position was not shared by patriarch Alexander and had therefore led to Arius’ excommunication. His doctrine did not suffer a drop in popularity though. Constantine greatly valued unity within the Christian church, but he seems to have seriously underestimated the impact of the “Arian question”. In a conciliatory letter to Arius and Alexander he for instance wrote that he considered the cause of the controversy to be “of a truly insignificant character, and quite unworthy of such fierce contention”. This is a perfect example of Constantine’s lack of theological knowledge. For him as an emperor and a man of action, the dispute about the nature of Christ was purely academical, whereas for many Christians in the east – and especially for their bishops and priests – it was an extraordinarily fundamental issue, which could even lead to outbursts of violence.
Constantine decided to send a mediator to the east to try and find a solution for the Arian question. This mediator was very likely bishop Ossius of Corduba, the emperor’s friend and confidant in religious affairs. Unfortunately Ossius was unable to force a breakthrough, after which Constantine decided to organise an important church council. This council was originally set to take place in Ancyra in Galatia, a region where none other than Saint Paul the Apostle had preached, but it was ultimately moved to Nicaea, just south of Nicomedia. On the 20th of May of the year 325, the Council of Nicaea was formally opened. According to Eusebius, who as bishop of Caesarea in Palestine was himself present at Nicaea, over 250 bishops participated in the council. The number of participants was even larger if all the priests, deacons and other attendants are included.
The great majority of the bishops and other church officials were from the east of the Roman Empire. Just five bishops from the Roman west participated, i.e. Ossius of Corduba, Marcus of Calabria, Caecilianus of Carthage, Domnus of Sirmium and the Gallic bishop Nicasius. The bishop of Rome, Pope Sylvester (314-335), confined himself to sending a number of priests, just like he had done with regard to the Council of Arelate in 314. According to Eusebius, the main reason was Sylvester’s advanced age. Christianity was certainly more developed in the Greek-speaking east than in the Latin west, but that can only partially explain the lack of enthusiasm among western bishops to participate in the council. Another factor must have been that a decision about the date for Easter had already been taken at the aforementioned Council of Arelate. Moreover, the Arian question seems to have been a typically “eastern” dispute. There were, by the way, also a couple of participants from outside the Roman Empire, including a Persian and a Gothic bishop.
Constantine himself presided over the council and held an opening speech in Latin. The choice of language was motivated by the fact that even in the predominantly Greek-speaking east Latin was the official language of government and law. As not all participants were proficient in Latin, Constantine’s speech had to be translated into Greek. The many discussions during the council must have been held exclusively in Greek. This was the first language of Constantine’s mother Helena, and the emperor himself had long served in the east under Diocletianus and Galerius, so he must have been perfectly bilingual. Eusebius claims that Constantine took an active part in the discussions, but given his lack of theological knowledge it is much more likely that he confined himself to chairing the meeting. The council took a number of important decisions. The relationship between the Father and Son was described as homo-ousios, “of the same substance” or “consubstantial”. This word made Arianism a heretical doctrine. The orthodox position was then codified in the Nicene Creed, which declared that Christ was “begotten, not made, consubstantial with the Father”.
The council furthermore condemned the quartodecimani, i.e. those Christians who celebrated Easter on the fourteenth day of the month of Nissan. This was one day before the Jewish holiday of Pesach, the feast commemorating the Hebrew exodus from Egypt. Since the Jews were accused of having committed a “heinous crime” – the crucifixion of the Messiah – the council no longer felt it right to use the Jewish calendar for calculating the date for Easter. From now on, Easter was to be celebrated every year on a Sunday, which was already the case in the western part of the Roman Empire. However, the exact method for calculating the right date remained a bone of contention until well after the council, as the date depended on the beginning of spring. The council moreover prohibited self-castration and took decisions about synods and ordaining priests. The bishops of Rome, Alexandria and Antioch were recognised as patriarchs, making them the highest-ranking officials in the Christian church. Contrary to popular belief, the clergymen gathered at Nicaea did not discuss the Holy Spirit or the Trinity, nor did they decide which books were to be considered canonical.
Unity and disunity
On 25 July, while the Council of Nicaea was still in session, Constantine celebrated his twentieth year on the throne, his Vicennalia. The next month the council was formally closed. The discussions had been heated at times. According to a later tradition there had even been a physical altercation, which saw bishop Nicholas of Myra – yes, Saint Nicholas – punch Arius in the face. Whether this story is true or not, the doctrine formulated by the Alexandrian priest did not lose any of its popularity. Arian Christians were a common sight not just within, but also outside the Empire. Most Christian Goths, Vandals and Longobards preferred Arius’ views on the relationship between the Father and the Son. This was an important factor in Arianism’s survival until well into the sixth century.
- Anonymus Valesianus, Origo Constantini Imperatoris 24;
- Aurelius Victor, De Caesaribus 41 (translated and annotated by H.W. Bird);
- Epitome de Caesaribus 41;
- Eusebius, Vita Constantini, Book 2.8, 2.68, 3.8, 3.18;
- Eutropius, Breviarium Historiae Romanae6;
- Zosimus, Historia Nova, Book 2.
- Adrian Goldsworthy, The Fall of the West, p. 178 and p. 188;
- Henk Singor, Constantijn, p. 353-378;
- Timothy Venning, A Chronology of the Roman Empire, p. 651-656.
 Zosimus, Historia Nova, Book 2.22.
 Eusebius, Vita Constantini, Book 2.8.
 Origo Constantini Imperatoris 5.24.
 Eusebius, Vita Constantini, Book 2.68.
 Eusebius, Vita Constantini, Book 3.8.
 Nicasius was bishop of Colonia Dea Augusta Vocontiorum, modern Die.
 Eusebius, Vita Constantini, Book 3.18.
 The council also spoke about the so-called Melitian schism in Egypt. Bishop Melitius of Lycopolis (modern Asyut) and his supporters opposed the readmission into the Church of Christians who during the persecutions had sacrificed, handed over sacred texts or otherwise denied their faith. The schism was in a way comparable to the conflict involving the Donatists in Africa.
 See the fourteenth-century author Petrus de Natalibus.