So, in this particular game I was kind of goofing off and I was going for a space race victory on the 3rd difficulty level I believe. Obviously I need to go higher. 
Anyway, I was very much in with the City States this game. I was allied with at least half of them, four of which I liberated from the evil Germans and English! The Maritime city state of Ragusa, my ally, had war declared on them by the Persians. I didn't much want to lose my benefits from Ragusa, but at the same time I wasn't really eager for an intercontinental war.
It just so happened that on the turn that Persia declared war on Ragusa, I had gotten the tech to make Infantry. Nobody else in the game had the tech to make such units, and I decided to start gifting Ragusa my infantry to defend him against the Persians.
They held off the Persians, but they were still locked in a permanent war. Then something happened that I never saw before... Ragusa conquered a Persian city.
Realizing that I could really help them push harder, I continued supplying them with infantry as I was able, and made a beeline toward Mechanized Infantry (where I was the first to get those as well), and gifted a couple of them as well.
Before you know it, Ragusa has the most technologically advanced army in the world (thanks to me) and ends up capturing several Persian cities (and razing 2 or 3). They even took the Persian capital!
I obviously could have taken out Persia myself, but I really enjoyed helping a city state conquer a rival through under the table deals, while still being at peace with them. Plus... getting all those free resources from the extra cities was quite nice as well.
Attached is a screenshot, submitted for your approval:
PS. This is my first ever post here... though I have lurked several years since joining... and even lurked several years before joining! I think I did it wrong... low count posters are supposed to hate Civ 5, right? ;-)

Anyway, I was very much in with the City States this game. I was allied with at least half of them, four of which I liberated from the evil Germans and English! The Maritime city state of Ragusa, my ally, had war declared on them by the Persians. I didn't much want to lose my benefits from Ragusa, but at the same time I wasn't really eager for an intercontinental war.
It just so happened that on the turn that Persia declared war on Ragusa, I had gotten the tech to make Infantry. Nobody else in the game had the tech to make such units, and I decided to start gifting Ragusa my infantry to defend him against the Persians.
They held off the Persians, but they were still locked in a permanent war. Then something happened that I never saw before... Ragusa conquered a Persian city.
Realizing that I could really help them push harder, I continued supplying them with infantry as I was able, and made a beeline toward Mechanized Infantry (where I was the first to get those as well), and gifted a couple of them as well.
Before you know it, Ragusa has the most technologically advanced army in the world (thanks to me) and ends up capturing several Persian cities (and razing 2 or 3). They even took the Persian capital!
I obviously could have taken out Persia myself, but I really enjoyed helping a city state conquer a rival through under the table deals, while still being at peace with them. Plus... getting all those free resources from the extra cities was quite nice as well.
Attached is a screenshot, submitted for your approval:
PS. This is my first ever post here... though I have lurked several years since joining... and even lurked several years before joining! I think I did it wrong... low count posters are supposed to hate Civ 5, right? ;-)