Actually believe it or not there are sometimes when that does happen in the game. Im always the player that sits back and let my power grow in economic and army wise while the AI fights each other. And i discover that when i dont form any packs or alliences with any of the AI and start taking over just one of the AIs planet in thier galaxy, that AI will start ingoring the other AI's and form alliences with them. Which is what i love because it starts to make the game more interesting .Im not sure if is still the case in diplomacy cause i havent played it that much but in enternchment it's exactly like that most of the time.

I'll get on my "Allied Victory" soap box and say that the AI would be best to stay allied with you.

If the AI thinks "this guy is to powerful, I should attach him now to ensure my own victory," to me this implies that there is a limit to the number of people who can win, which is what I've been pushing for.  See my "Allied Victory" post.

Currently, every single player can win (not a single loser), so there is no logical reason for the AI to "think" about betraying you.  Afterall, you can both win.

This is a good idea, and would make most logical sense if there were a limit on the number of possible winners in a game.

Place your votes here for victory conditions and other improvements to Diplomacy.

Consolidated list of improvements