Born in 250AD in the Balkans, he first comes to notice during campaigns against Palmyra and subsequently was made governor of Dalmatia. In 288 he was made Pretorian Prefect to the co-emperor Maximian and campaigned along the Rhine border. In 293 he was appointed Caesar (second in command / vice-Emporeror) of the Western Territories taking command of Hispania (Spain), Gaul (France) and Britannia. This was rather a poisoned chalice since Northern Gaul and Brittainia had been in revolt since 286 and was claimed by the rebel leader Carausius. Constantius defeated both Carausius and Allectus, who had assumed command of the rebels on the former’s death. He set about replacing the rebel administration and introduced the administrative reforms of Diocletian. He continued to divide his time between Britain and the Rhine frontier.
In May 305 he took over from Maximian as Augustus of the West and was joined in Gaul by his son Constantine, who many had expected to be named Cesaer in his father place, but this instead had been given to Severus, a nominee of Galerius, the Augustus of the East. Father and Son crossed over to Britain and campaigned against the Picts north of Hadrians Wall. They retired to York for the winter, but Constantius was taken ill and died. The army, rejecting the Western Cesaer, Severus, acclaimed Constantine as Emporer. In a shrewd political move, Constantine quickly accepted the role of Cesaer to Severus, thus avoiding war and giving him time to prepare for a campaign that would eventually see him control the whole empire.