I am really struggling to understand how the whole diplomacy points/diplomacy victory system works.

As I try to learn, I just played a single-player game with 4 players:  me+AI vs. two AI's, with teams locked.  I began playing per my usual strategies and started noticing that the two enemy AIs' diplomacy points were rising very quickly compared to mine and my partners.  Again, since I don't nearly at all understand what factors contribute to these rising points, I had no idea what to do.  After winning battle after battle, suddenly, the game was over - they won with the diplomatic victory.

So what am I missing here?  What is the point?  In such a scenario is it just a matter of sending envoy after envoy to your partner, and gifting each other as quickly as possible, regardless of other factors in play?  Similarly, once an enemy's diplomacy points are rising, and nearing victory, is there ANYthing I can do to slow or reverse the trend?

I'm really really just not getting this aspect of the game.  We really REALLY need some sort of tutorial or FAQ or somethign to explain this.