Is there some reason or special game technical conditions other than having the alliance tech to keep you from forming an alliance with another civilization? I don't mean they won't agree to it, I mean the game is not allowing me to even make the offer.

I'm in a position to win a diplomatic victory by forming an alliance with the one remaining major alien civilization. There were four to begin with, and I have eliminated one by conquest and have negotiated alliances with the other two. So it is not that I can't form alliances, just not with the remaining civ. We both have the Alliances tech. I gave them the Alliances tech many turns back, along with some other gifts. Our relations are warm and they are very weak, but it is not that they are turning me down, it is that Alliance is not shown on the Trade Screen so I can't even offer Alliance to them. Can someone help me out here with a possible reason for this? Or is there some way to offer an alliance other than on the Trade Screen?

I would like to end this game now with a diplomatic victory rather than take the time to eliminate this last civ by conquest because I've learned so much from all my questions here and the excellent replies. I'm anxious to try a new game now that I'm smarter.

Thanks for your replies.