Answer: In 452 Atilla the Hun (born in Pannonia, a province of the Roman Empire [present-day Transdanubia, Hungary], around 406), threatens Rome with a large army invasion from northern Italy. Pope Leo I rides out from Rome, meets with him, and after some negotiation, Atilla withdrawals. However, in 452 Rome is sacked by the Vandals, an east-Germanic tribe. Facing this invasion, all Pope Leo can do is work to prevent even more death and destruction.

It is interesting to note that the Vatican Post Office has issued two sets of stamps covering the subject of Pope Leo talking with Attila. The first stamps are Scott numbers 150 and 152 which were part of the 1951 1500th Anniversary Of The Council of Chalcedon series. These stamps were depicted from a fresco painted by Raffaello from 1513 to 1514 that is in the Heliodorus Room of the Vatican Museum. The second set of stamps is Scott numbers, 301, 302, and 303 which are the 1961 stamps issued to commemorate the Fifteenth Centenary Of The Death Of Pope Leo The Great. The design of the second set of stamps is based on a high-relief by Alessandro Algardi that is above St. Leo's tomb in St. Peter's Basilica in Rome. (See pictures images below.)


Stamps Showing Pope Leo I Meeting with Atilla
based on the Raphael painting
Scott 150 & 1523

By Raphael - Web gallery of Art: [2] Art Renewal Center, Public Domain, Link

Stamps Showing Pope Leo I Meeting with Atilla
based on tomb in St Peter Basilica
Scott 301-303

By Peytonlow - Personally taken photography, CC BY-SA 3.0, Link

References
•  Anonymous Vatican City Philatelic News Vatican Notes Volume X Issue 2 September - October 1961 Page 2
•  Bolaffi's 1969 Specialized Stamp Catalog - Roman States and Vatican City 1969 - (see Design Notes for stamps issues)
• Gordon Kerr, "452" & "455" Timeline of the Popes RW Press Limited copyright 2013
• William Wonneberger, "The Vatican Postage Stamps" National Philatelic Museum Volume 7 Number 1 page 87 (See section on Counsel Of Chalcedon. This publication was produced for the 1954 Marian Year Philatelic Exhibition held in Philadelphia, September 22 - November 5, 1954.)