On 23 October 425 the emperor Valentinian III was installed as ruler of the western half of the Roman empire. The act was a triumph for the Theodosian dynasty, which had lost its grip on the west following the death of Valentinian’s uncle, the emperor Honorius, on 15 August 423, and, at first sight, a remarkable demonstration of imperial unity. The young Valentinian (born on 2 July 419) had been taken to Constantinople by his mother Galla Placidia even before Honorius died. Valentinian’s father, Flavius (Fl.) Constantius, had done much to reconstitute the western empire in the 410s. He then married Galla Placidia (Honorius’ sister) on 1 January 417 at the start of his second consulship, and had himself declared co-emperor of the west in February 421. He died the following September, before he could extract recognition of his self-promotion from Constantinople. His death let loose an extended power struggle in the west, which at first centred on controlling the inactive Honorius.

Placidia and Valentinian had fled east in the course of these disputes in 422. When, after Honorius’ death, power was seized by a high-ranking member of the western bureaucracy, the notarius John, the eastern emperor, Theodosius II, eventually decided to back Valentinian and the cause of dynastic unity. Hence, in spring 425, a large eastern force – combining fleet and field army – moved west, and despite the capture of its commander, Ardaburius, quickly put an end to the usurper.