The Monophysites had been making a come back and when the Patriarch Flavian of Constantinople would not tolerate them, their leader Eutyches turned to Rome. This controversy pushed the Emperor to call the Council of Ephesus in 449 to which Leo I sent representatives to lay out the official Catholic position on the equal human and divine natures of the person of Christ. Pope Leo made it clear that the position of the Bishop of Rome was final and binding, and he was fortunate enough to have the support of the temporal powers in this as well as the Roman Emperor Valentinian III issued an edict in 445 recognizing the spiritual supremacy of the Pope and giving him the support of the empire in dealing with heretics and others who rebelled against official Church teaching. This was the case when Pope Leo dealt with the French dissident Bishop Hilary of Arles.
Any way that one measures a great pontiff, St Leo fits the bill. He was a stone wall in standing up to heresies, a staunch defender of Catholic doctrine, a thoughtful theologian, a compassionate shepherd and a man of courage who was willing to stand up to even the most fierce world leaders of his time without blinking. He was a gifted speaker and left behind 96 sermons which testify to this fact, urging Christians to acts of sacrifice, to fasting, to give to the poor, to put away all of their old pagan beliefs (some of which continued to linger) and effectively promoted what would later be called the 'social kingship of Christ'. However, his most famous moments came in what today would be known as "foreign relations".
In Leo's time, the greatest enemy Rome, and in fact all of Western Civilization faced, was the cruel, cunning and ambitious barbarian chieftan Attila the Hun. After an initial reign of terror and destruction across both the Eastern and Western Empires Attila met his first defeat in France by a Roman and Gothic coalition under General Flavius Aetius. However, Attila was now back with a vengance, having the promise of half the empire made by a foolish Roman princess who offered to marry the chieftan as dowry and rather than focus on Gaul, Attila headed for Italy. By this time, the Roman legions were reduced to almost nothing and Rome had no strength left with which to confront Attila. Therefore, in what can only be described an an incredibly heroic act, Pope St Leo went out himself to stop the one known as "the scourge of God".
Attila had just completed a 3-year siege by taking the city of Aquileia and was ready to move on Rome itself when the Pope confronted him near Mantua in 452. The unarmed churchman faced the most feared and vicious barbarian ruler and appealed to him to spare the Eternal City and return to his own country. What exactly happened will remain a mystery, but Attila reportedly said that as St Leo spoke he saw a terrifying figure in white robes holding a sword, silently warning him to heed the Pope's request and withdraw from Italy. To the shock of the entire Roman world, Attila did just that, returning his massive army of feared warriors back to Hunnish territory where he died only a short time later.
Pope St Leo was placed in a similar position again in 455 when the dreaded Vandals under Gaiseric also attacked Rome. This time, the Pope was not able to save the city completely, but his efforts did save Rome from considerable destruction by obtaining from Gaiseric the promise that certain areas would be spared. Through all of his efforts, diplomatic as well as religious, his sermons, and his endless work to preserve the purity of Church doctrine, it is easy to see why it was said that, "Peter has spoken through the mouth of Leo" and easy to see why he was made a saint, a Doctor of the Church and finally given the title of "St Leo the Great". He was the first pope to be buried in St Peter's Basilica and was moved several times, and on one occasion as late as 1607 his body was found to be incorrupt. He died on November 10, 461 after reigning for 21 years.