Valentinian III (419-455) was the son of Emperor Constantius III, and nephew of former Emperor Honorius through his mother, Galla Placidia. He was also cousin to Theodosius II, Eastern Roman emperor. When his father died in 421, barely seven months into his co-rule, Valentinian was only two years old, and he fled with his mother and sister to Constantinople from the unwanted attentions of his uncle Honorius.

When Honorius died in 423 and the usurper Joannes took power, Theodosius recognised Valentinian as the true emperor of the West, and placed him on the throne in 425, aged only six. Due to his age, his mother ruled as regent in his stead, desperately attempting to stablise the Western Roman Empire and negotiating with the Huns. The empire continued to lose more territory however, and internal instability wracked the empire constantly. Valentinian finally became emperor in 437, but true power remained in the hands of others.

Attila the Hun's invasion of the Western Roman Empire, at the behest of Valentinian's own sister Honoria, devastated much of the western provinces and was only just stopped at the gates of Rome. Valentinian was eventually assassinated in 455 after murdering one of his powerful advisors, Aetius, the year previous. Overall, Valentinian's reign is marked by the continued dismemberment and decline of the Western Roman Empire.