OK, I admit it. This is a joke that has to be explained to almost everybody. Lots of people have heard of Attila the Hun but don’t know much more than that he was one bad dude. Fewer people know who Pope Leo was except that he must have been a pope or something.
The facts are…way back in 452 the King of the Huns was ravaging and pillaging Europe and was determined to have his way with Rome which was undefended because the Roman army was licking its wounds from a previous battle with Attila a year earlier in Gaul. The Romans had managed to thwart Attila’s march through western Europe but had to have the help of several Germanic tribes who feared the Huns more than they hated the Romans. After that big battle those tribes moseyed off leaving the Romans in no shape to face Attila alone.
So, instead, the Romans sent out the Pope. After Leo met with Attila, the Hun said to heck with it and went back home. What really happened is mostly conjecture. Since the Romans were the only ones writing stuff down all we have are their versions which include, among other things, the appearance of saints and angels. Therefore, I offer another possible rationale. It might be as equally plausible as any other except for the fact that those types of playing cards weren’t around yet.