I have a somewhat different impression, since countries mostly ally with neighbours and countries with similar culture, religion and/or government in the games I have played.
If it was guaranteed, I would have experienced world wars as well, which I have not, so I would suggest fiddling with the algorithms the AI uses to decided whether to go to war or not instead of setting an artificial barrier for when a call to arms can be made. It should be a decision based on potential threat and gain, not "we can not join this war because we are only allies of the belligerent's allies". Besides; even if you put that barrier in place, there is no guarantee that the AI will refrain from DOWing outright if it senses weakness just like a human player would, nor should there be.
