I'm wondering about this too. I really don't like how, currently, states of peace, war, and alliances with non-teammates are shared between all team members. It should be perfectly possible to be allied with someone and not be at war with their enemies. This might carry a diplomatic penalty, sure, but I don't think it should force a state of war. It's also far too easy to exploit. Losing a war to a computer player and they're not willing to make peace with you? Simply have your teammate make peace with their teammate.
