There are some general findings about diplomacy in TW:
-The bigger you are province-wise the harder it is to get an alliance.
- (RTW-specific) Allies seem more inclined to backstab and attack you than a country that only has trade relations.
- (MTW-specific) If you and your enemy share allies the one who wins the first battle keeps his allies. In Rome, shared allies seem to side with the AI, no matter what.