@Caramtchir “casus belli” means: an act or situation provoking or justifying war. Her thinking is correct, she has no provocation to wage war yet, nor does she desire to break her relationship with the village.
In-game terms you are correct, right about any situation can be called an "occasion for war", but for "roleplay" and strategy purposes our MC must balance the interests of her swarm with her own. She is pretty much aware of “casus belli”, and she likely is searching for an idea that meets her's and the swarm's interests.
( I'm not saying that "she probably shouldn't be saying 'casus belli' out loud." Or that she isn't taking her sweet old time doing anything else. I'm just saying her decision making is faithful to the “casus belli” concept.)
