Constantius. I. Surnamed Chlorus(the Pale), a Roman emperor, father of Constantine the Great, born about A. D. 250, died at York, England, in July, 306. During his short reign the emperor Carus purposed to adopt Constantius, then governor of Dalma-tia, in place of his own son, the unworthy Carinus; but his sudden death in 283 prevented the execution of this project. Constantius afterward served with distinction, especially under Diocletian. In 292 the joint emperors Diocletian and Maximian made over a part of the actual exercise of government to their favorite generals, Galerius and Constantius, with the title of Csesars, obliging each of them to divorce his wife and marry a daughter of one of the emperors. Constantius, who was appointed by Diocletian, received the government of Britain, Gaul, and Spain, his capital being the modern Treves. He defeated the Franks on the lower Rhine, the Alemanni on the upper Rhine, and the usurper Allectus in Britain. He governed with great humanity, and gave special protection to the Christians. In 305, when Diocletian and Maximian abdicated, Constantius and Galerius were proclaimed Augusti, the precedence being assigned to the former.

He died while on an expedition against Scotland, 14 years after he was created Caesar, and 15 months after receiving the title of Augustus. II. A Roman emperor, second son of Constantine the Great by his second wife Fausta, born at Sirmium in Pannonia, Aug. 6, 317, died at Mopsocrene in Cilicia, Nov. 3, 361. After the death of Constantine the empire was apportioned between his three sons, Constantius, then 20 years of age, receiving Thrace and the countries of the East, including Egypt. He, however, seized upon Constantinople, which had been assigned to his elder brother Constantine, and put to death two of his uncles and seven of his cousins; but it was soon agreed between the three brothers that Constantinople should be restored to Constantine. He waged a protracted war against the Persians, which prevented him from taking any part in the struggles between his brothers in the West. Constantine was killed in 340 and Constans in 350. Constantius thereupon marched against Magnentius, who had been proclaimed in Gaul and Italy, and against Vetranio in Illyricum. They offered to compromise by acknowledging him as emperor of the East,. with the preeminence in rank.

He refused, having been, as he said, directed to do so by a vision of his father, the great Constantine. He offered, however, to acknowledge Vetranio as his colleague and equal, on condition that he would abandon Magnentius. A conference was appointed, but the soldiers of Vetranio deserted him, whereupon he abandoned all his claims, and was assigned an honorable abode at Prusa. Oonstantius then marched against Magnentius, whom he encountered and defeated in the bloody battle of Mursa (now Eszek) in Pannonia (Sept. 28, 351), and thus became sole emperor. He had in the mean time given the title of Caesar to his cousin Gallus, and sent him to Antioch to govern the East; but in 354 he caused him to be beheaded, appointed Julian, the brother of Gallus, as Caesar, and sent him to Gaul, where he waged a successful war against the Ale-manni and Franks. Growing jealous of the renown of Julian, he ordered the recall of the legions from Gaul. A revolt ensued, and the troops proclaimed Julian emperor. Constantius moved against him, but died on the way, and Julian, afterward known as "the Apostate," whom it is said he appointed his heir on his deathbed, was peacefully acknowledged by the whole empire.

Constantius is known for his zealous endeavors to establish a uniformity of faith throughout the empire. He favored the Arians, expelled Athanasius from Alexandria, persecuted the Catholics and the Novatians, and endeavored to put down pagan rites and ceremonies. III. A general of the Roman emperor Honorius, died in 421. He defeated Gerontius and a soldier named Constantine who had revolted and been proclaimed as emperor in Britain in 411; was made consul of Rome in 414 and 417; married Placidia, sister of Honorius, and was proclaimed joint emperor with him in 421, but died seven months after. His son became the emperor Valentinian III.