Allies - can't be allied with civs that are enemies of each other

[GMO]

Currently when you form an alliance with a civilization the enemies of that civilization say they don't like it including civilizations that you are allied with. So you can be allied with civilizations that are enemies of each other. This can be unrealistic in some cases.
To make it more realistic some civs should give you an ultimatum - either ally with me or ally with the other civ but you can't be allied with both at the same time. You'll have to make a choice to support one civ or group of civs, or choose another civ or group of civs.

 

[WChrisMullen]

I find it makes for good "kingmaker" scenarios, where you can sit out on a war between two major powers but tip the balance in favor of one if it looks like one side is losing. War tends to stagnate development, so while they're sitting on cities that have been bombarded into oblivion, you're building spaceship parts.

 

[Halcyan2]

Currently when you form an alliance with a civilization the enemies of that civilization say they don't like it including civilizations that you are allied with. So you can be allied with civilizations that are enemies of each other. This can be unrealistic in some cases.
To make it more realistic some civs should give you an ultimatum - either ally with me or ally with the other civ but you can't be allied with both at the same time. You'll have to make a choice to support one civ or group of civs, or choose another civ or group of civs.

Occasionally, AI's already do exactly this. They may ask you to denounce another player (who could be your Friend) and if you don't, they end up denouncing you instead.

If you look at AI relationships with each other, most of the time, when two AI's don't get along, their friends end up picking one side or another. But not always - there occasionally are some AI's (and/or you) who can manage to maintain Friendships with two civs that hate each other.

 

[Matthew.]

Yeah, they can pick sides. I tend to be proactive and if I have plans to attack a specific Civ at one point, I try and denounce them ASAP. Declaration of Friendship seems to be largely time-based and as long as you scout aggressively, you will meet the other civilizations before your denounce target does, and they will end up siding with you. Doing this I've even been able to maintain friendly relations after completely wiping out the enemy Civ.

Turns out denouncements aren't completely worthless