Constantius I [Flavius Valerius Constantius; called Constantius Chlorus] (250?–306), Roman emperor, was born in the Balkan province of Illyricum on 31 March, probably in 250. It is alleged that he was related to the emperor Claudius Gothicus (Claudius II) (r. 268–70), though this may be a later invention to enhance the imperial family's antecedents. His parentage is unknown but the subsequent use of names in the family suggest that his father may have been a Flavius Delmatius and his mother possibly Julia Constantia. His nickname Chlorus (‘the Pale’) is not recorded before the sixth century but is likely to be a contemporary reference to his personal appearance. He embarked on a military career the nature of which suggests that he had powerful patronage, serving successively as a protector (that is, a member of the immediate military entourage of the emperor, probably Aurelian), unit commander, and a governor of the Adriatic province of Dalmatia, probably in 284 or 285. Less reliable sources suggest that he served as a general under the emperor Probus (r. 276–82). His military ability drew him to the attention of the joint emperors Diocletian and Maximian and he served as praetorian prefect to Maximian from 288 to 293, presumably campaigning in northern Gaul where a peasants' revolt had erupted.
In 293 new constitutional arrangements were implemented by which the emperors made preparation for their planned eventual retirement by creating two designated successors; Constantius was promoted to imperial rank as Caesar and heir apparent to the western ruler, Maximian. His immediate task was to recover parts of Gaul and Britain which had been wrested from imperial control by the usurper Carausius (r. c.286–293). An attempt to recover Britain by Maximian had failed and, as a consequence, hostile forces were well established on the continental mainland. Gesoriacum (Boulogne), the principal stronghold of Carausius in Gaul, was besieged. Denied relief by a fleet from Britain by a mole which blocked access to the River Liane, the city fell. Carausius was assassinated by Allectus, his principal minister, who succeeded him and maintained hostilities. Following further campaigning, Gaul was cleared of hostile garrisons. In 296 an invasion force comprising two separate armies descended on southern Britain. One army, led by the praetorian prefect Asclepiodotus, landed in the area of modern Hampshire and defeated the army led by Allectus in person. The usurper fell in the conflict. The other force, led by Constantius, failed to make a landfall owing to adverse weather conditions, being delayed by contrary winds and fog and only arriving, via the Thames, in time to prevent the looting of London by fleeing remnants of the usurper's forces. None the less, a triumphant entry into London, the provincial capital, is recorded both by contemporary historians and by commemorative medals struck for issue to participants in the campaign. A copy now in the British Museum of a medal found at Arras in northern France bears depictions of Constantius and of his rescue of London. After the recovery of the island, widespread administrative changes were introduced which were probably overseen by the Caesar. Later events suggest that, possibly after an initially poor reception, Constantius's conduct immediately after the suppression of the revolt gave him a degree of popular support from the people of Britain which was reflected in the reign of Constantine.
Marriage, or a less formal liaison, with Flavia Julia Helena produced one son, later to be the emperor Constantine I (the Great). On elevation to imperial rank, Constantius divorced Helena and entered a dynastic marriage with Theodora, the stepdaughter of the emperor Maximian, with whom he had six children. Helena, later converted to Christianity by her son, embarked on a life of pilgrimage and church building; she was later canonized. Enmity between the two branches of Constantius's family culminated in the massacre of Theodora's kin following the death of Constantine the Great in 337.
After the campaign in Britain, Constantius was occupied for four years in warfare on the Rhine frontier. In the persecution of Christianity which was inaugurated by Diocletian and Maximian in 303, Constantius is credited with not having implemented the full rigour of the law in territory under his control. The story may owe more to the subsequent career of Constantine than to a coherent administrative decision by Constantius prompted by religious sympathy.
In May 305 the emperors Diocletian and Maximian retired from office, handing the succession to Constantius in the west and Galerius in the east. Shortly after his accession events in Britain needed imperial intervention. A campaign in northern Britain was called for against the Picts, a generic name for the tribes of the highlands of Scotland. According to contemporary sources, possibly echoing accounts of campaigns in the region conducted in the first century AD by the governor Gnaeus Julius Agricola and by the emperor Septimius Severus in the first decade of the third century, Constantius is credited with having penetrated to the extreme north of Scotland. Victory was achieved in 305 since in that year Constantius took the additional names Britannicus Maximus for the second time, the first having been after the defeat of Allectus.
Constantius died at Eburacum (York) on 25 July 306, sufficiently long after the assumption of Britannicus Maximus to suggest that his last illness was prolonged and prevented him from returning to London, much less to the imperial capital at Trier. Constitutional arrangements, made at the resignation of Diocletian and Maximian, designated Severus II as successor to Constantius, but the presence of Constantine with his father in Britain ensured, probably with paternal connivance, that Constantine should be proclaimed emperor on the death of his father. Based on the power of the army of Britain, Constantine forced his constitutional recognition on the legitimate emperors, eventually achieving sole imperial power in 324.