He affirmed the two natures of Christ at the Council of Chalcedon. Defended Rome from barbarian invasions

Today we celebrate the sanctity of one of the greatest pontiffs to have honored the Roman See. His pontificate lasted for more than 21 years and has been without any doubt one of the most important in the history of the Church. The times in which Pope Leo (390-461) lived were very difficult: constant barbarian invasions, the gradual weakening of imperial authority in the West and the long, drawn-out social crisis forced the Bishop of Rome to play an important role in civil and political events. —Leo the Great was a tireless champion and upholder of the Roman Primacy, presenting himself as the Apostle Peter's authentic heir.