Civilization – Militaristic city-state allies gifting unique units

I accomplished this once. Some things I remember:

• I had to weaken/kill off some particular number of units, 5 maybe?
• I think I had to do that fairly close to the city state.
• Once I did, they were my allies for a LONG time after. The relations boost was substantial.

You might have missed something else entirely. I know in the case of barbarian missions you must be the one to finally KILL the units, weakening them in any amount is simply ignored. Maybe that applies here?

On another note, gifting units provides only a minor boost in relations, nothing compared to fulfilling any request or any gift of gold. You should not bother unless you really don't want the unit you are gifting. (I let military city-states spawn units for me always, and simply give them right back if I don't need them.)

If you kill quite a number of units of a city state and/or keep trespassing on their territory enough to get your score very very low over an extended period of time, they eventually will be convinced that you are so evil it's not worth considering peaceful coexistence, so they will be in perpetual war with you. You will not be able to buy their friendship and relations will not heal by themselves over time.

Actually that doesn't mean too much, though. If you go near them, they'll attack you, and if you leave cities undefended near them, they'll capture those. Apart from that, they won't hunt you across the map and they won't make others hate you as well. If you just leave them alone now and make their area taboo to your units you can still get along with the other city states just fine.

You might choose to have one perpetual warring city-state next to your army outfitter because city-state units train your troops longer than barbarians do.