Marcus Flavius Valerius Constantius (c. 250 – 25 July 306), also called Constantius I, was a Roman emperor from 305 to 306—and was father of Constantine the Great, the first Christian emperor of Rome. He was one of the four original members of the Tetrarchy established by Diocletian, first serving as caesar from 293 to 305 and then ruling as augustus until his death. The nickname "Chlorus" (Ancient Greek: Χλωρός, lit. 'the Pale') was first popularized by Byzantine-era historians and not used during the emperor's lifetime.
As a soldier of humble origin, Constantius had a distinguished military career and rose to the top ranks of the army. Around 289, he divorced Helena, the mother of Constantine the Great, to marry a daughter of Emperor Maximian, and in 293 was added to the imperial college by Maximian's colleague Diocletian. Assigned to rule Gaul, Constantius defeated the usurper Carausius there and his successor Allectus in Britain, and campaigned extensively along the Rhine frontier, defeating the Alamanni and Franks. When the Diocletianic Persecution was announced in 303, Constantius ordered the demolition of churches but did not actively hunt down Christians in his domain. Upon becoming senior emperor in May 305, Constantius launched a successful punitive campaign against the Picts beyond the Antonine Wall. He died suddenly at Eboracum (York) in July the following year.
After Constantius's death, the army, perhaps at his own instigation, immediately acclaimed his son Constantine as emperor. This act contributed to the collapse of the Diocletianic tetrarchy, sparking a series of civil wars that ended only when Constantine finally united the whole Roman Empire under his rule in 324. According to the Oxford Classical Dictionary, "Constantinian propaganda bedevils assessment of Constantius, yet he appears to have been an able general and a generous ruler". His descendants, the Constantinian dynasty, ruled the Empire until the death of his grandson Julian in 363.
Constantius's birthday was 31 March; the year is unknown, but his career and the age of his eldest son imply a date no later than c. 250. Constantius was a Thracian or an Illyrian. He was born in Moesia Superior (later Dacia Ripensis), a Roman province on the south bank of the Middle Danube. 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. The same source also gives Claudius the nomina "Flavius Valerius" to strengthen his connection to Constantius. Modern historians suspect this maternal connection to be a genealogical fabrication created by his son Constantine I, and that his family was of humble origins. Constantine probably sought to dissociate his father's background from the memory of Maximian.
Constantius was a member of the Protectores Augusti Nostri under the emperor Aurelian and fought in the east against the secessionist Palmyrene Empire. While the claim that he had been made a dux under the emperor Probus is probably a fabrication, 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. It has been conjectured that he switched allegiances to support the claims of the future emperor Diocletian just before Diocletian defeated Carinus, the son of Carus, at the Battle of the Margus in July 285.
In 286, Diocletian elevated a military colleague, Maximian, to the throne as co-emperor of the western provinces, while Diocletian took over the eastern provinces, beginning the process that would eventually see the division of the Roman Empire into two halves, a Western and an Eastern portion. By 288, his period as governor now over, Constantius had been made praetorian prefect in the west under Maximian. 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. To consolidate the ties between himself and Emperor Maximian, Constantius married the emperor's daughter, Theodora.
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. The eastern and western provinces would each be ruled by an augustus, supported by a caesar. Both caesares had the right of succession once the ruling augustus died.
At Mediolanum (Milan) on 1 March 293, Constantius was formally appointed as Maximian's caesar. He adopted Diocletian's nomen (family name) "Valerius", and, being equated with Maximian, also took on "Herculius". His given command consisted 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 21 May 293 at Philippopolis (Plovdiv). Constantius was the more senior of the two caesares, and on official documents he always took precedence, being mentioned before Galerius. Constantius's capital was to be located at Augusta Treverorum (Trier).
Constantius's 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 (Boulogne-sur-Mer). Carausius was then assassinated 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 Julius Asclepiodotus, Constantius's 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 Londinium (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's 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 Britannia Superior into Maxima Caesariensis and Britannia Prima, while Flavia Caesariensis and Britannia Secunda were carved out of Britannia Inferior. He also restored Hadrian's Wall and its forts.
Later in 298, Constantius fought in the Battle of Lingones (Langres) against the Alemanni. 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 thereby strengthening the defences 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's attention.
From 303 – the beginning of the Diocletianic Persecution – Constantius began to enforce the imperial edicts dealing with the persecution of Christians, which ordered the destruction of churches. 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. Eusebius denied that Constantius destroyed Christian buildings, but Lactantius records that he did.