Flavius Valerius Constantius (March 31 c. 250–July 25 306) was an emperor of the Western Roman Empire (305–306). He was commonly called Chlorus (the Pale) an epithet given to him by Byzantine historians. He was the father of Constantine I and initiator of the Constantinian dynasty.
In 293 the emperor Diocletian created the Tetrarchy, dividing the Roman Empire into Western and Eastern portions. Each would be ruled by an Augustus, supported by a Caesar. Diocletian became Augustus of the Eastern empire, with Galerius as his Caesar. Constantius was appointed Caesar to the Western Augustus, Maximian, and married Theodora, Maximian's stepdaughter. They had six children. Constantius divorced his first wife (or concubine), Helena, by whom he already had a son, Constantine. Helena was probably from Nicomedia in Asia Minor. He was given command of Gaul, Britain and possibly Hispania.
In 293, Constantius defeated the forces of Carausius, who had declared himself emperor in Britain and northern Gaul in 286, near Bononia. Carausius was killed by his rationalis Allectus, who took command of Britain until 296, when Constantius sent Asclepiodotus, a prefect of the Praetorian Guard, to invade the island. Allectus was defeated and killed, and Roman rule in Britain restored.
Also in 296, Constantius fought a battle against the Alamanni at the city of Lingonae (Langes) in Gaul. He was shut up in the city, but was relieved by his army after six hours, and defeated the enemy. He defeated them again at Vindonissa (Windisch, Switzerland), thereby strengthening the defenses of the Rhine frontier.
Diocletian and Maximian stepped down as co-emperors in 305, due to Diocletian's poor health, and the Caesars, Constantius and Galerius, became co-emperors. Constantius ruled the western empire, Galerius the eastern. Severus and Maximinus were appointed Caesars. Constantine, who had hoped to be a Caesar, joined his father's campaigns in Gaul and Britain. Constantius died in Britain, at York, in 306, and Constantine was declared emperor by the army.