|Pope Leo is the one on the white horse, just left of the cross, under the angels.|
In this, Attila was neither delusional nor barbarous, but rather just a chump. Honoria, it seems, had been betrothed to a Roman senator by her brother. She didn't think much of the match, so she wrote to Attila, proposing the marriage and the division of the Empire. She sent her ring as a token of her sincerity. Attila, like any noble prince, rode to rescue the winsome princess from her unfortunate marriage to Senator Gasbagus. Valentinian might have killed his sister, but their mother intervened to see that she was only exiled. But the damage was done -- the Huns were on the peninsula, ready for war.
Leo the Great rode north to -- well, negotiate is not the right word, since he had nothing to offer. I guess he rode north to persuade Attila to go home. And the strange thing is, the Hun King listened, then turned his army around and went home.
The reasons suggested for this vary from the fantastic to the pragmatic. Let's start with the latter and work back to the former.
1. The Huns were running low on supplies. Moreover, plague broke out in their camp.
2. The Eastern Roman Emperor, Marcian, had sent an army to relieve Italy.
3. Aetius, another plundering king, was threatening Attila's possessions on the Catalupian plains. Attila had to get back and settle that business.
4. Attila was reminded at some point that Alaric had sacked Rome but died shortly after. It was not safe to earn the disfavor of Heaven.
5. Leo was just a persuasive fellow. No one knows what he said, but there's no denying his ability to persuade others. Was his eloquence a divine gift, and Attila's receptivity also divinely ordained?
6. The vision of a huge man in priestly robes brandishing a gleaming, razor-honed sword was visible only to Attila as Leo spoke. [This story is courtesy of Paul the Deacon's eighth century Historia Romana.]
7. Any combination of the above reasons.
Whatever the reason(s), Attila listened politely and then packed up and went home. He did not give up his claim to half the Western Empire, nor to Honoria, but he did feel compelled to leave at that moment, a pretty miraculous result.