Valentinian I (; 32117 November 375), sometimes called Valentinian the Great, was Roman emperor from 364 to 375. He ruled the Western half of the empire, while his brother Valens ruled the East. During his reign, he fought successfully against the Alamanni, Quadi, and Sarmatians, strengthening the border fortifications and conducting campaigns across the Rhine and Danube. His general Theodosius defeated a revolt in Africa and the Great Conspiracy, a coordinated assault on Roman Britain by Picts, Scoti, and Saxons. Valentinian founded the Valentinianic dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire.
Early life
Valentinian was born in 321 at Cibalae (now Vinkovci, Croatia) in southern Pannonia into a family of Illyrian origin. Valentinian and his younger brother Valens were the sons of Gratianus (nicknamed Funarius), a military officer renowned for his wrestling skills.
Gratianus was promoted to comes Africae in the late 320s or early 330s, and the young Valentinian accompanied his father to Africa. However, Gratian was soon accused of embezzlement and retired. Valentinian joined the army in the late 330s and later probably acquired the position of protector domesticus. Gratian was later recalled during the early 340s and was made comes Britanniae. After holding this post, Gratianus retired to the family estate in Cibalae.
In 350, Constans I was assassinated by agents of the usurper Magnentius, a commander who proclaimed himself emperor in Gaul. Constantius II, older brother of Constans and emperor in the East, promptly set forth towards Magnentius with a large army. The following year the two emperors met in Pannonia. The ensuing Battle of Mursa Major resulted in a costly victory for Constantius. Two years later he defeated Magnentius again in southern Gaul at the Battle of Mons Seleucus. Magnentius, now realizing the futility of continuing his revolt, committed suicide in August that year; making Constantius sole ruler of the empire. It was around this time that Constantius confiscated Gratianus' property, for supposedly showing hospitality to Magnentius when he was in Pannonia. Despite his father's fall from favour, Valentinian does not seem to have been adversely affected at this time, making it unlikely he ever fought for the usurper. It is known that Valentinian was in the region during the conflict, but what involvement he had in the war, if any, is unknown.
Service under Constantius and Julian
The conflict between Magnentius and Constantius had allowed the Alamanni and Franks to take advantage of the confusion and cross the Rhine, attacking several important settlements and fortifications. In 355, after deposing his cousin Gallus but still feeling the crises of the empire too much for one emperor to handle, Constantius raised his cousin Julian to the rank of Caesar. With the situation in Gaul rapidly deteriorating, Julian was made at least nominal commander of one of the two main armies in Gaul, Barbatio being commander of the other. Constantius devised a strategy where Julian and Barbatio would operate in a pincer movement against the Alamanni. However, a band of Alamanni slipped past Julian and Barbatio and attacked Lugdunum (Lyon). Julian sent the tribunes Valentinian and Bainobaudes to watch the road the raiders would have to return by. However, their efforts were hindered by Barbatio and his tribune Cella. The Alamanni king Chnodomarius took advantage of the situation and attacked the Romans, inflicting heavy losses. Barbatio complained to Constantius and the debacle was blamed on Valentinian and Bainobaudes, who were cashiered from the army.
With his career in ruins, Valentinian returned to his new family estate in Sirmium. Two years later his first son Gratian was born by his wife Marina Severa. Valentinian's actions and location become uncertain around this time, but he was likely exiled. Theodoret says that this was because he had reacted angrily when a pagan temple attendant sprinkled water on him, saying "I am not purified, but defiled", and striking the priest.
In 357 Valentinian was a tribunus of cavalry, possibly the Joviani in Roman Gaul. In either 359/360 or 360/361 he was in Mesopotamia, and by 362 he was comes and tribunus of the Cornuti. Valentinian was then exiled to Thebes, in the Thebaid of Roman Egypt. Under Jovian he was promoted to tribunus of the scholae secundae scutariorum, a formation of the elite Scholae Palatinae.