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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. Diocletian divided the administration of the Roman Empire into two halves, a Western and an Eastern portion. Each would be ruled by an Augustus, supported by a Caesar. Both Caesars had the right of succession once the ruling Augustus died.
At Milan on March 1, 293, Constantius was formally appointed as Maximian’s Caesar. He adopted the names Flavius Valerius and was given command of Gaul, Britannia and possibly Hispania. Diocletian, the eastern Augustus, in order to keep the balance of power in the imperium, elevated Galerius as his Caesar, possibly on May 21, 293 at Philippopolis. Constantius was the more senior of the two Caesars, and on official documents he always took precedence, being mentioned before Galerius. Constantius’ capital was to be located at Augusta Treverorum.
Constantius’ first task on becoming Caesar was to deal with the Roman usurper Carausius who had declared himself emperor in Britannia and northern Gaul in 286. In late 293, Constantius defeated the forces of Carausius in Gaul, capturing Bononia. This precipitated the assassination of Carausius by his rationalis (finance officer) Allectus, who assumed command of the British provinces until his death in 296.
Constantius spent the next two years neutralising the threat of the Franks who were the allies of Allectus, as northern Gaul remained under the control of the British usurper until at least 295. He also battled against the Alamanni, achieving some victories at the mouth of the Rhine in 295. Administrative concerns meant he made at least one trip to Italy during this time as well. Only when he felt ready (and only when Maximian finally came to relieve him at the Rhine frontier did he assemble two invasion fleets with the intent of crossing the English Channel. The first was entrusted to Asclepiodotus, Constantius’ long-serving Praetorian prefect, who sailed from the mouth of the Seine, while the other, under the command of Constantius himself, was launched from his base at Bononia. 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. Constantius in the meantime occupied London, saving the city from an attack by Frankish mercenaries who were now roaming the province without a paymaster. Constantius massacred all of them.
Constantius remained in Britannia for a few months, replaced most of Allectus’ officers, and the British provinces were probably at this time subdivided along the lines of Diocletian’s other administrative reforms of the Empire. The result was the division of Upper Britannia into Maxima Caesariensis and Britannia Prima, while Flavia Caesariensis and Britannia Secunda were carved out of Lower Britannia. He also restored Hadrian’s Wall and its forts.
Later in 298, Constantius fought in the Battle of Lingones (Langres) against the Alamanni. 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. In 300, he fought against the Franks on the Rhine frontier, and as part of his overall strategy to buttress the frontier, Constantius settled the Franks in the deserted parts of Gaul to repopulate the devastated areas. Nevertheless, over the next three years the Rhine frontier continued to occupy Constantius’ attention.
In 303, Constantius was confronted with the imperial edicts instituted by Diocletian dealing with the persecution of Christians. 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. Of the four Tetrarchs, Constantius made the least effort to implement the decrees in the western provinces that were under his direct authority, limiting himself to knocking down a handful of churches.
Between 303 and 305, Galerius began maneuvering to ensure that he would be in a position to take power from Constantius after the passing of Diocletian. In 304, Maximian met with Galerius, probably to discuss the succession issue and Constantius either was not invited or could not make it due to the situation on the Rhine. Although prior to 303 there appeared to be tacit agreement among the Tetrarchs that Constantius's son Constantine and Maximian's son Maxentius were to be promoted to the rank of Caesar once Diocletian and Maximian had resigned the purple, by the end of 304 Galerius had convinced Diocletian (who in turn convinced Maximian) to appoint Galerius’s nominees Severus and Maximinus Daia as Caesars.
Diocletian and Maximian stepped down as co-emperors on May 1, 305, possibly due to Diocletian's poor health. Before the assembled armies at Milan, Maximian removed his purple cloak and handed it to Severus, the new Caesar, and proclaimed Constantius as Augustus. The same scene played out at Nicomedia under the authority of Diocletian. Constantius, notionally the senior emperor, ruled the western provinces, while Galerius took the eastern provinces. Constantine, disappointed in his hopes to become a Caesar, fled the court of Galerius after Constantius had asked Galerius to release his son as Constantius was ill. Constantine joined his father's court at the coast of Gaul, just as he was preparing to campaign in Britain.
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. After retiring to Eboracum (York) for the winter, Constantius had planned to continue the campaign, but on 25 July 306, he died. As he was dying, Constantius recommended his son to the army as his successor; consequently Constantine was declared emperor by the legions at York.