Civilization – Civ V Multiplayer – Change turn mode while in a game

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.)

Sounds like you were unfortunate to not spot the issue earlier; probably the best thing you could have done at that stage would be to fortify your borders, hope to deflect the first wave (when it eventually comes) and then counterattack, charge in and capture and hold a few of their cities (thus making them weaker). Although if you were aiming for a cultural victory you would need to avoid annexing any, otherwise this will make your social policies cost more.

In my experience Montezuma and Augustus are, by default, quite aggressive when played by the AI; if you find an AI playing either near to you, I feel it is best to plan ahead to stop them expanding too much and have sufficient defence on that border - or instead try to make sure you get friendly with them so they're on your side instead.

Also, I believe entering in to a pact of Secrecy with other Civ's against a threatening Civ earlier in the game would have helped prevent the other Civ's becoming allies with the threat later on, by ensuring they have a "bad" relationship with the threat. But I don't have hard evidence for this.