6 At the request of the court of Ravenna the Pope sent St. Leo to Gaul to settle a dispute between the patrician Aetius and the prefect Albinus. It was while Leo was engaged on this mission that Sixtus III died. Recognizing Leo's unrivalled theological learning and practical wisdom in diplomacy and the conduct of affairs, the Roman Church could think of no more worthy candidate for Christ's vicarious power on earth than this deacon.
Source: Aeterna Dei Sapientia (Vatican.va)