The Roman Dominate: The Tetrarchy
by: P. Dionysius Mus

Diocletianus and Maximianus, Augusti, with Galerius and Constantius I, Caesars: Diocletianus defeated the Emperor Carinus (283-285) in a fight at the river Margus in Dalmatia and thus became sole ruler over the Roman Empire. In November 285 he granted Maximianus, a very loyal servant, the title 'Caesar', meaning he also became Emperor, so that Diocletianus could more easily solve the problems on the Danube river. Maximianus accepted Diocletianus as an Emperor above him and not next to him, and was even given the title 'Augustus' in 286. In 293, after his campaign on the Danube, Diocletianus decided that each one of them would appoint a 'Caesar' who would rule with them and act as their successor. Maximianus appointed Constantius, and Diocletianus chose Galerius. Diocletianus reorganised the empire in twelve administrative units, called 'dioceses', and created four new capitals at the borders of the empire: Trier, Milan, Thessalonica and Nicomedia.

Galerius and Constantius, Augusti, with Severus II and Maximinus Daia, Caesars: On May 1st 305 Maximianus and Diocletianus left the throne in favour of their two Caesars (who became then Augusti), and those new emperors (Constantius I and Galerius) appointed their own Caesars on the same day: Severus II and Maximinus Daia.

But the new emperors' sons, Constantinus and Maxentius, felt left out because their fathers appointed two other Caesars. When Constantius, who was nicknamed 'Chlorus' ("very pale"), died, possibly from leukaemia, Constantinus made his own soldiers call him 'Augustus' (but as it was agreed before, Severus II was officially recognised as Augustus). Maxentius, the other son, started a revolution in Italy following an extra tax collection, supported by the people there and the Praetorian Guard. He had himself named emperor over Italy, Sicily, Sardinia and North Africa and appointed his father Maximianus (who had ruled together with Diocletianus) as co-ruler. Severus II attacked them, but was defeated and killed in Italy. Galerius, who invaded Italy, was also defeated, but was forced to flee.

Galerius and Licinius, Augusti, with Maximinus and Constantinus, Caesars: Now there really were too much candidates for the throne, and Galerius decided to hold a meeting in Carnutum (November 308), led by the old Diocletianus. Here they agreed on this new tetrarchy: Licinius (an army commander) as Augustus in the West, Galerius as Augustus in the East, Constantinus as Caesar in the West, and Maximinus as Caesar in the East.

But the old Maximianus, who acted as Constantinus' counsellor after the meeting, could not hide his ambitions and named himself again emperor. He took refuge in Massilia, but when Constantinus came marching on demanding for justice, they opened the gates and Maximianus killed himself. In 310 Galerius, sick with cancer, was succeeded by his Caesar, Maximinus. But at the same time Maxentius still ruled Italy after his revolution, so Constantinus advanced towards Rome, where he defeated Maxentius in battle; the latter fled following his defeat and drowned in a river.

Constantinus, with and then against Licinius, rival Augustus, and Valens, rival Caesar: Next the Augustus Licinius marched to the East and defeated Maximinus, the new Eastern Augustus, who then fled and poisoned himself. Until 316 Licinius and Constantinus kept working together very well, but when they finally met each other in battle, Licinius was defeated. He fled and appointed Valens as co-ruler for his military qualities. A new battle followed, forcing Constantinus to sign a peace treaty. But in 324 Constantinus definitively defeated the forces of Licinius, who hung himself in 325. From that moment on Constantinus was the sole ruler of the Roman Empire; he made Constantinople his new capital and became known to history by the name of Constantine the Great.
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