1. The civil strives after the death of Galerius

Constantine's way to the imperial throne and the recreation of an empire governed by a single emperor, after the Tetrarchy instigated by Diocletian (284-305), was copious and bloody. The period after the death of Galerius, Diocletian's successor, was marked by a series of civil wars aiming at supremacy. Constantine, who had succeeded his father Constantius Chlorus at the government of Britain almost with a cue in 306,¹ deliberately pursued the formation of family ties with the emperor Maximian² and his son Maxentius by marrying at a second wedding the former's daughter, Fausta. After Galerius' death in 311, there were three remaining candidates for the throne: Valerius Licinianus or Licinius -whom Galerius had appointed his co-emperor, and governor of Illyricum, Thrace and the Danube provinces-, Galerius' nephew Maximinus Daia -whom his uncle had appointed Caesar in 305 in charge of the eastern part of the empire- and Constantine. However, Maxentius had also aspirations to the throne, and he probably had bitter feelings for being left out -and perhaps rightly so.³ With Maximian's death Maxentius turned openly against Constantine, whom he accused as a murderer and rebel, ordering the erasion of his name from all the inscriptions of Italy at least.

In order to face Maxentius successfully, Constantine joined sides with Licinius, since the territories which Maxentius had taken over belonged technically to him. Licinius, however, could not deal with the usurper at that point, as he was preoccupied with the defection of Maximinus Daia in the East. The arrangement between Licinius and Constantine was sealed with an engagement, that of Licinius with Constantine's half sister, Constantia.

After a victorious advance in Italian territories, Constantine met finally Maxentius in battle on February 28th, 312, at the Milvian Bridge, outside Rome. Shortly before this decisive battle, Constantine saw the famous vision of “in this you win”, which is considered as the incentive for his conversion to Christianity.⁴ After the battle and the death of Maxentius, Constantine remained sole master in Europe, although the Italian territory, which he had freed from the usurper, belonged to Licinius.

2. Constantine and Licinius as co-emperors

The two augusti initially seemed to be on good terms, despite the agreement between Licinius and Maximinus in 312 on the Bosporus, after which the latter continued to govern the eastern provinces, to persecute the Christians and to treat the subjects with violence. In the spring of 312 Maximinus broke this agreement, crossed the Bosporus and conquered Byzantion and the surrounding areas. Licinius, who already discussed with Constantine the possibility of issuing a decree of religious tolerance, moved against him and crushed Maximinus. After his victory he entered Nicomedia triumphantly and on the 13th of June of the same year he issued the edict of religious freedom thus giving the Christians the chance to express their faith publicly. The edict is known under the name “Edict of Milan”, since it was in that city that its content had been agreed upon between Constantine and Licinius.⁵ The latter had thus taken under his control the entire eastern part of the empire. In order to eliminate every bit of resistance by Maximinus' supporters he imposed a regime of terror and started executing ministers of his usurper predecessor, as well as members of the latter's family. He didn't show respect not even to the widow of Galerius and daughter of Diocletianus, Valeria, and his mother, Prisca, who were executed.

3. The first clash

When he secured his power in the East, Licinius started plotting against Constantine. However his plot was revealed, but Constantine avoided taking further measures. In the beginning of summer 314 Licinius took one more provocative action against his co-emperor. He ordered that all statues of Constantine in Aemona (modern Ljubliana), a city right on the western borderline of his territory, should be destroyed. Constantine considered this provocation as the proclamation of war and returned from Gaul, where he was stationed with his entire army. The battle took place at Cibalis, in the valley of river Sarros, on the 8th of October.⁶ Constantine won and forced Licinius to sign a peace treaty and to adhere to him all his Balkan possessions, but he acknowledged his supremacy on Asia, Egypt and Libya. Fearing a new provocation by Licinius, Constantine transferred his capital to the east, from Sirmium to Serdica (modern Sofia).

4. The second clash

For a few years the relations between the two emperors remained on a fragile balance. In 320, however, Licinius showed again a provocative behaviour. Despite the edict of tolerance which he had signed together with Constantine, he took some anti-Christian measures, by abolishing councils in his area of jurisdiction and by removing bishops and clerics from their positions. The answer of Constantine was immediate and well-targeted: he abolished the tradition of appointing one consul from the East and one from the West, he proclaimed as consuls for the year 320 himself and his first-born son Crispus, whereas in the next year he proclaimed as consuls his two sons, Crispus and Constantine, still a minor. At the same time he ordered the construction of a huge war fleet, which he aimed at using against his rival. Licinius, on the other hand, prepared also his army and fleet. However, until 322 the two enemies waited for an instigation to start the war.

The pretext was given when Constantine, in the course of a military expedition against the Sarmatians, led his troops to Thrace. Licinius marched against him with a large army, consisting mainly of inhabitants of Asia Minor and Syria. The battle took place at the outskirts of Hadrianople on the 3rd of July 323. Despite his defeat Licinius used Byzantion as a defense point and proclaimed his rival destitute from imperial power. Constantine waited patiently in Hadrianople until his fleet came. The next stage of the war took place at the Sea of Marmara. Constatine's fleet under the leadership of Crispus, after a battle which lasted for two days, crushed the fleet of Licinius, led by Avantos, and headed towards the city of Byzantion. Licinius, terrified, crossed over to the Asian shores. Constantine pursued him and achieved another great victory in Chrysoupolis (modern Üsküdar) on the 18th of September. Licinius then fortified himself in his capital, Nicomedeia, but Constantine's final victory was imminent and Licinius' wife persuaded him to surrender. In fact she went to her half-brother herself in order to mediate for the clauses of the surrender. Constantine accepted the surrender of his rival and deported him in Thessalonica; he also deported his prime minister, Marcus Martianus, to whom Licinius had conferred the title of Augustus, to Cappadocia.

Yet, Constantine's respect for his rivals lasted only for a few months. Under undefined conditions the two men were executed shortly afterwards.