Constantine, joined by Roman legions from Spain, has by May 408 made Arles his capital, where he appoints Apollinaris, the grandfather of Sidonius Apollinaris, as prefect.

In the summer of 408, as the Roman forces in Italy assemble to counterattack, Constantine has other plans.

Hispania is a stronghold of the House of Theodosius and loyal to the ineffectual emperor.

Fearful that several cousins of the emperor Honorius will organize an attack from that direction while troops under Sarus and Stilicho attack him from Italy in a pincer maneuver, Constantine strikes first at Hispania.

He summons his eldest son Constans from the monastery where he is dwelling, elevates him to caesar, or junior emperor, and sends him with the general Gerontius towards Hispania.