This study investigates the roles played by the barbarian general Sigisvult and the Arian bishop Maximinus in western imperial politics during the reign of Valentinian III. In c. 426/7 Sigisvult was sent to North Africa to subdue the rebel Count Boniface. He was accompanied by Maximinus, who, in the course of the campaign, engaged in a celebrated debate with Augustine. Maximinus helped achieve a non-violent settlement that returned Boniface and his German troops to the imperial fold. Sigisvult then returned to Italy and was appointed Patrician and Master of Soldiers after the assassination of Felix in 430. In 435, however, this position was transferred to Fl. Aëtius, who spent most of his time campaigning in Gaul. In 440, the Vandals attacked Sicily, and Sigisvult was placed in charge of the defence of the Italian coast. At the same time, an Arian bishop Maximinus, apparently the one who had debated with Augustine, was in contact with the Vandals. He may have planted disinformation that Sebastianus, the son of the now-deceased Boniface, was about to invade Africa. This resulted in a Vandal withdrawal, and both Italy and Sicily were saved. Sigisvult last appears in the mid-440s, and his subsequent demise, perhaps in the early 450s, may have resulted in the breakdown of a precarious balance of power and the murders of Aëtius and Valentinian in 454 and 455, which hastened the demise of the Western Roman Empire. In an appendix it is suggested that although Boniface called in Vandal auxiliaries in 427, he was not responsible for the Vandal crossing in 429.