Really, alliances should be dynamic. That way the game will always be changing, who has what power, through changes in alliances. I think this is quite advanced Diplomacy play though, and I find a lot of players struggle with it, often jumping to accusations "back-stabbing" and getting upset.
Precisely! That's it.
Diplomacy play on a high level.
Typed by a highly strategic player.*
*: Sorry, couldn't resist... ;)