Emperor Flavius Valerius Constantius was born on March 31st around 250 AD. It is known that his family was from the province of Illyricum. He served in the army as a protector, then as tribunus and finally as a praeses Dalmatiarum. Approximately between 270 and 280 AD his son Constantine (later the Great) was born from his first marriage with Helena. In 288 AD he became Praetorian Prefect of the western emperor Maximianus Herrculius. He was commonly called Chlorus (the Pale), an epithet passed to us from Byzantine historians.
On 1 March 293 Diocletian appointed Galerius as his Caesar (junior emperor) in the east and Constantius as the Caesar of Maximianus Herculius in the west. Both Caesars were also given the right of succession. Constantius Chlorus was one of the original four Tetrarchs and was destined to replace Maximianus as emperor after twenty years. Diocletian's plan was for there to be two Augusti, or emperors, and two Caesars, or emperors in training. Each of the four would make his headquarters in a different city, two in the eastern half of the empire, two in the western half.
It was due to this imperial adoption that Constantius' family name now changed from Julius to Valerius. In order to strengthen the dynastic relationship between himself and Herculius, Constantius put aside his wife Helena and married Theodora, the stepdaughter of Maximianus Herculius. The marriage with Theodora produced six children: Flavius Dalmatius, Julius Constantius, Hannibalianus, Constantia, Anastasia, and Eutropia. To further strengthen his bond with Galerius and Diocletian in the east, Constantius allowed Galerius to keep his first son Constantine as a hostage to ensure Constantius' good behavior.
In the remainder of the time that he was a Caesar, Constantius spent much of it engaged in military actions in the west. In the summer of 293 Constantius expelled the troops of the usurper Carausius from northern Gaul. After Constantius' attack on Bononia (Boulogne), Carausius was murdered. At the same time he dealt with the unrest of the Germans. In 296 he invaded Britain and put down the revolt of the usurper Allectus. Between 300 and 305 A.D. the Caesar campaigned successfully several times with various German tribes. For example In 298 he overthrew the Alamanni in the territory of the Lingones (Langres) and strengthened the Rhine frontier.
During the persecution of the Christians in 303 he behaved with greater humanity then his colleagues who rigidly enforced the persecution. On 1 May 305 Diocletian, at Nicomedia, and Maximianus Herculius, at Mediolanum (Milan), divested themselves of the purple, probably because of the almost fatal illness that Diocletian contracted toward the end of 304. Diocletian forced Maximianus to abdicate. Constantius and Galerius, as their Caesars, then donned the purple, with Severus II and Maximinus Daia as the new Caesars. The retired Augusti, Diocletian and Maximianus then returned to private life.
Constantius, as had his predecessor, ruled in the west, while Galerius and Daia ruled in the east. Almost as soon as he was appointed Augustus, he demanded the return of his son Constantine (Galerius had virtually 'inherited' this guest from his predecessor Diocletian). He explained that a campaign against the Picts, who were invading the British provinces, required both his own and his son's leadership. Galerius, evidently under pressure to comply or to admit that he was holding a royal hostage, conceded and let Constantine go. Constantine caught up with his father at Gesoriacum (Boulogne) in early AD 306 and they crossed the Channel together. Soon after Constantius died at Eburacum (York) on 25 July 306 with his son at his side.