This act contributed to the collapse of the Diocletianic tetrarchy, sparking a series of civil wars which only ended when Constantine finally united the whole Roman Empire under his rule in 324.

According to the unreliable Historia Augusta he was the son of Eutropius, a nobleman from the province of Moesia Superior, and Claudia, a niece of the emperors Claudius Gothicus and Quintillus.

[15] Modern historians suspect this maternal connection to be a genealogical fabrication created by his son Constantine I,[16] and that his family was of humble origins.

[18] While the claim that he had been made a dux under the emperor Probus is probably a fabrication,[19][20] he certainly attained the rank of tribunus within the army, and during the reign of Carus he was raised to the position of praeses, or governor, of the province of Dalmatia.

[24] Throughout 287 and into 288, Constantius, under the command of Maximian, was involved in a war against the Alamanni, carrying out attacks on the territory of the barbarian tribes across the Rhine and Danube rivers.

[18] By 293, Diocletian, conscious of the ambitions of his co-emperor for his new son-in-law, allowed Maximian to promote Constantius in a new power sharing arrangement known as the Tetrarchy.

[30] Constantius spent the next two years neutralising the threat of the Franks who were the allies of Allectus,[31] as northern Gaul remained under the control of the British usurper until at least 295.

[31] Only when he felt ready (and only when Maximian finally came to relieve him at the Rhine frontier)[34] did he assemble two invasion fleets with the intent of crossing the English Channel.

[35] The fleet under Asclepiodotus landed near the Isle of Wight, and his army encountered the forces of Allectus, resulting in the defeat and death of the usurper.

[36] Constantius in the meantime occupied Londinium (London),[37] saving the city from an attack by Frankish mercenaries who were now roaming the province without a paymaster.

[34] Constantius remained in Britannia for a few months, replaced most of Allectus's officers, and the British provinces were probably at this time subdivided along the lines of Diocletian's other administrative reforms of the Empire.

[17] The campaign was avidly pursued by Galerius, who noticed that Constantius was well-disposed towards the Christians, and who saw it as a method of advancing his career prospects with the aging Diocletian.

[45] Of the four Tetrarchs, Constantius made the least effort to implement the decrees in the western provinces that were under his direct authority,[46] limiting himself to knocking down a handful of churches.

[24] Before the assembled armies at Mediolanum, Maximian removed his purple cloak and handed it to Severus, the new caesar, and proclaimed Constantius as augustus.

[51] In 305, Constantius crossed over into Britain, travelled to the far north of the island and launched a military expedition against the Picts, claiming a victory against them and the title Britannicus Maximus II by 7 January 306.

Eusebius's Life of Constantine claims that Constantius was himself a Christian, although he pretended to be a pagan, and while Caesar under Diocletian, took no part in the Emperor's persecutions.

[60] Similarly, the History of the Britons traditionally ascribed to Nennius[61] claims the inscribed tomb of "Constantius the Emperor" was still present in the 9th century in the Roman fort of Segontium (near present-day Caernarfon, in North Wales).

[62] David Nash Ford credited the monument to Constantine, the supposed son of Magnus Maximus and Elen, who was said to have ruled over the area prior to the Irish invasions.