How to break enemy alliances

[Jaywalker-Jack]

I noticed in a campaign where I have a protectorate of my own, they sometimes give me tribute, and sometimes don't. Does anybody know what exactly happens ? I suspected maybe a tribute once a year (so every four turns) but it didnt seem to follow this pattern. But then, that could be because the protectorate was bankrupt.

EDIT: On topic, I find a good way to break an enemy alliance is to take a core territory from one enemy, and present it as a gift to the other. War usually ensues.