Constantius I (Constantius Chlorus)kənstăn´shəs [key], c.250–306, Roman emperor (305–6). A career general, he gave up Helena to marry Theodora, the daughter of Maximian. He was made caesar (subemperor) under Maximian in 293 and gained prestige when his forces defeated the rebel Carausius. He went to Britain in 296, where he put down a rebellion of Carausius' successor, Allectus. Returning to Gaul, he defeated the Alemanni in 298. His vigor and his moderation made him popular with the people of the colonies as well as with his soldiers. The two emperors, Diocletian and Maximian, abdicated in 305, and Constantius and Galerius became emperors. The next year, however, Constantius died at York. On his death the imperial throne was claimed by his son Constantine (Constantine I), but the office was long contested.
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