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Galerius (Caius Galerius Valerius Maximinianus) (gəlēr´ēəs), d. 310, Roman emperor (305–10). Diocletian appointed him caesar for the eastern part of the empire in 293 (Constantius I was caesar of the West). He had to conduct hard campaigns in Pannonia and Asia. Defeated by the Persians in 296, he retrieved his reputation by a resounding victory over them in 297. On the abdication of Diocletian and Maximian in 305, he and Constantius succeeded as emperors. Galerius tried to increase his power, and after Constantius died in 306 he recognized Severus (d.307) as coemperor in the West. Severus and he attempted without success to put down the claims of Maxentius. After they were defeated and Severus was captured, Galerius had Diocletian approve the appointment of Licinius as emperor of the West. Constantius' son Constantine (Constantine I) and Maximin (d.313) then both claimed power. Galerius died before the confusion was eliminated by the victory of Constantine. Galerius had prompted the persecution of Christians under Diocletian but issued (309) an edict of toleration shortly before his death.

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