Valentinian III (; 2 July 41916 March 455) was Roman emperor in the West from 425 to 455. Made emperor in childhood, his reign over the Roman Empire was one of the longest, but was dominated by powerful generals vying for power amid civil wars and the invasions of Late antiquity's Migration Period, including the campaigns of Attila the Hun.
He was the son of Galla Placidia and Constantius III, and as the great-grandson of Valentinian I () he was the last emperor of the Valentinianic dynasty. As a grandson of Theodosius I (), Valentinian was also a member of the Theodosian dynasty, to which his wife, Licinia Eudoxia, also belonged. A year before assuming the rank of augustus, Valentinian was given the imperial rank of caesar by his half-cousin and co-emperor Theodosius II (). The augusta Galla Placidia had great influence during her son's rule. During his early reign Aetius, Felix, and the comes africae, Bonifatius all competed for power within the western empire. Eventually Aetius would defeat Felix and Bonifatius. Aetius would go on to campaign against the many Germanic tribes invading the empire.
During Valentinian's reign the Huns invaded the Roman Empire. Eventually Aetius would defeat the Huns at the Battle of the Catalaunian Plains. Once the Huns returned, Pope Leo I and two other senators convinced Attila to leave. Valentinian himself killed Aetius, and in response Aetius's bodyguards assassinated Valentinian. Valentinian's reign was marked by the ongoing collapse of the western empire.
Family and infancy
Valentinian was born in Ravenna, the capital of the Western Roman Empire, as the only son of Galla Placidia and Constantius III. His mother was the younger half-sister of the western emperor Honorius (), while his father was at the time a patrician and the power behind the throne.
Through his mother, Valentinian was a descendant both of Theodosius I, who was his maternal grandfather, and of Valentinian I, who was the father of his maternal grandmother. It was also through his mother's side of the family that he was the nephew of Honorius and first cousin to Theodosius II (the son of Honorius' brother Arcadius), who was eastern emperor for most of Valentinian's life. Valentinian had a full sister, Justa Grata Honoria, who was probably born in 417 or 418 (the history of Paul the Deacon mentions her first when mentioning the children of the marriage, suggesting she was the eldest). His mother had previously been married to Ataulf of the Visigoths, and had borne a son, Theodosius, in Barcelona in 414; but the child had died early in the following year, thus eliminating an opportunity for a Romano-Visigothic line.
When Valentinian was less than two years old, Honorius appointed Constantius co-emperor, a position he would hold until his death seven months later. As a result of all these family ties, Valentinian was the son, grandson, great-grandson, cousin, and nephew (twice over) of Roman emperors.
In either 421 or 423, Valentinian was given the title of nobilissimus by Honorius, although this title was not initially recognized in the eastern court of Theodosius II. After the death of his father in 421, Valentinian followed his mother and his sister (Justa Grata Honoria) to Constantinople, when court intrigue saw Galla Placidia forced to flee from her half-brother, the emperor Honorius, and the young Valentinian went to live at the court of his cousin Theodosius II.
Early Reign (423–437)
Caesar
In 423, Emperor Honorius died, and his primicerius notariorum, Joannes took power in Rome. To counter this threat to his power, Theodosius belatedly recognised Valentinian's father as augustus (emperor) and nominated the 5-year-old Valentinian caesar (heir-apparent) for the West in 23 October 424. Theodosius also betrothed him to his own daughter Licinia Eudoxia (whom Valentinian would eventually marry in 437 when he was 18). It was only in the following year, after Joannes had been defeated in a combined naval and land campaign, that Valentinian was installed by the eastern patricius et magister officiorum, Helion, as augustus in Rome, on 23 October 425, at the age of six.
Augustus
Given his minority, the new augustus ruled under the influence of his mother Galla Placidia, one of whose first acts was to install Felix as the magister utriusque militiae in the west. This period was marked by a vigorous imperial policy and an attempt to stabilize the western provinces as far as the stretched resources of the empire could manage.