>>32868805A number of HiE stories have been written about this. Some get into HFY territory.
And I get the impression--and it's only vague, a good storyteller can go in other directions--that the cute little poni, Princesses and all, are, down deep, plains grazers with a deeply ingrained flight reflex. They are herd creatures and they're not wired in a way that makes hunting or war instinctive for them. The fact that they live in a world with multiple sentient species suggests they might have had to learn about war. The power the Princesses personally wield makes them like gods of legend, but at base, by instinct--I think it'd frighten even them, on some level.
I've written before about the idea some authors seem to have that, were humans and poni to have contact, the main problem might be that humans would tend to squee themselves to death within minutes. It seems unlikely to me that this would be the main problem. Our visitors would--unavoidably--see things they didn't like, maybe even some of the same things about our world we don't like when we look at it.
Some people have written stories about Sunbutt & Co. being so horrified and frightened that Celestia went a bit nuts and attempted to go total magical genocide on humanity. I wouldn't go that far--but I'd expect them to be very, very cautious after the third or fourth public appearance of the Princesses got a visit from some kind of suicide bomber or crazy would-be assassin shouting about "witchcraft." Just giving them uncensored Internet access might do it, and letting them see information about humanity's recent history.