information:classification, structural:expansion
Leo and Attila

After the indecisive outcome of the Battle of Chalons in 451, Attila invaded Italy in 452, sacking cities such as Aquileia and heading for Rome. He allegedly demanded that the sister of the reigning Emperor Valentinian III be sent to him with a dowry. In response, the emperor sent three envoys to negotiate with Attila: Gennadius Avienus, one of the consuls of 450, Memmius Aemilius Trygetius, the former urban prefect, and Leo. Little is known of the specifics of the negotiations, as a result of which Attila withdrew. Most ancient and medieval historians celebrated Leo's actions, giving him all the credit for this successful embassy. According to Prosper of Aquitaine, who was alive at the time of the event, Attila was so impressed by Leo that he withdrew. Another near-contemporary was the historian Priscus who records that Attila was dissuaded from attacking Rome by his own men because they feared he would share the fate of the Visigothic king Alaric, who died shortly after sacking the city in 410. Paul the Deacon, in the late 8th century, relates that an enormously huge man dressed in priestly robes and armed with a sword, visible only to Attila, threatened him and his army with death during his discourse with Leo, and this prompted Attila to submit to his request.

Writing in the early 20th century, the religious skeptic John B. Bury remarked:

The fact of the embassy cannot be doubted. The distinguished ambassadors visited the Hun's camp near the south shore of Lake Garda. It is also certain that Attila suddenly retreated. But we are at a loss to know what considerations were offered him to induce him to depart. It is unreasonable to suppose that this heathen king would have cared for the thunders or persuasions of the Church. The Emperor refused to surrender Honoria, and it is not recorded that money was paid. A trustworthy chronicle hands down another account which does not conflict with the fact that an embassy was sent, but evidently furnishes the true reasons which moved Attila to receive it favourably. Plague broke out in the barbarian host and their food ran short, and at the same time troops arrived from the east, sent by Marcian to the aid of Italy. If his host was suffering from pestilence, and if troops arrived from the east, we can understand that Attila was forced to withdraw. But whatever terms were arranged, he did not pretend that they meant a permanent peace. The question of Honoria was left unsettled, and he threatened that he would come again and do worse things in Italy unless she were given up with the due portion of the Imperial possessions.

Leo's intercession could not prevent the sack of the city by the Vandal King Genseric in 455, but murder and arson were repressed by his influence. The Pope and members of his clergy, went to meet the invader to implore him to desist. While the Vandals plundered the city, the gesture nevertheless prevented Rome from being burned and assured that the Basilicas of St Peter, St Paul and St John, in which part of the terrified population sought refuge, were spared. Leo assisted in rebuilding the city of Rome, restoring key places such as Saint Peter's.