GALERIUS (Galerius Valerius Maximianus
- In November 285, Diocletian made Maximian Caesar (later Augustus), with control of the western provinces.
- On 1 March 293, Diocletian adopted Galerius Valerius Maximianus as son and Caesar, while Maximian did the same for his praetorian commander Julius Constantius. Diocletian as Augustus with Galerius as Caesar ruled the east, while Maximian as Augustus ruled the west with Constantius as Caesar. This system of four rulers was known as the Tetrarchy.
- To cement the arrangement, Galerius married Diocletian's daughter Valeria in June 293.
- In 296 the Persians seized control of Armenia from the Roman client-king Tiridates and began to move towards Antioch. Galerius advanced against them but was defeated in Mesopotamia. Diocletian blamed him publicly for attacking the Persians with an inferior force.
- Galerius returned in 297 with reinforcements from the Danube armies. He achieved total victory, gaining new territories along the Upper Tigris.
- Between 297 and 304 Diocletian and Galerius issued a string of anti-Christian edicts, which persecuted Christians who were tortured and killed if they refused to recant. Christian writers have blamed Galerius.
- In 304, Diocletian and Maximian abdicated. Constantius and Galerius became the new senior emperors, and Maximinus Daia (Maximinus II) and Severus (II) were appointed to take their place as Caesars.
- When Constantius died at York on 25 July 306, his army proclaimed his son, Constantine, "Augustus", and senior emperor in the west in succession to his father. When Galerius heard this, he sent back a purple robe as recognition of Constantine as Caesar, not Augustus. He promoted Severus the rank of Augustus instead.
- Maxentius, the son of Maximian, was determined to get into the act and had himself proclaimed emperor in Rome in October 306.
- Galerius ordered Severus to get rid of Maxentius. Severus arrived at the walls of Rome but his armies deserted him and he abdicated and was killed on 16 September 307.
- Diocletian was briefly brought out of retirement in November 308 for an imperial conference at Carnuntum near Vienna. The result was a new tetrarchy. Licinius, another Illyrian army officer and friend of Galerius, was appointed Augustus in the west, in place of Severus. Constantine and Maximinus were given the title of filius Augusti, Maxentius was declared a public enemy and Maximian, whom Maxentius had created co-ruler, was forced to resign. Under pressure from Constantine and Maximinus, Galerius promoted them to full Augustii, early in 309.
- On 30th April 311, Galerius died of a terrible cancer that ate him up. The Christians blamed the wrath of God, which Galerius himself believed as he rescinded the anti-Christian laws he had promulgated eight years before.