Quote Originally Posted by Ludens
I meant: does this apply to the computer as well, or is just that your allies/protectorates always side with someone else? Or is it related to faction size?
It is related only to who attack who first, unfortunately. It is one of the major complaints in the diplomatic flaws that protectorates would betray their masters if the allies of those protectorates got attacked by the masters [It brought a lot of WTF!?]. Being a computer or a human player doesn't matter, so is the faction "strength."

So having a protectorate who's allied to many factions can tie your hands up from many ambitious expansionistic wars.

But I still love having protectorates, nonetheless. I like cultural diversity IF I rule the world. Their benefits to the player, however, are superficial. Apart from keeping them from ever breaking their "alliance" with you by conventional means (like attacking you or cancelling alliance), their provinces are counted as yours in the victory condition. It is rare that you can push them to fight wars for your cause.