I think an allied AI factors in your military rating when making negotiations. How does this work? Are they just added together, or what? An enemy AI was extorting this one, but as soon as I allied with it they went to war. Also my Ally went to war with me, despite the fact that we were "close" and half their eco depended on my trade. I was weak because of a war they had forced me into, but they were weaker. They couldn't count on their ally's help because they were allied with me too! is this a bug?
If the relation screen says close you haven't allied yet. If you open the diplomacy screen you will have the option for 'Alliance". Once that happens you will stay allied until you break it our they declare war on someone and you dishonor the alliance by not going to war as well. You can be allied with two of the AI civs and they can be at war with each other.