I am interested in this idea.

I have some thoughts about the idea:

A conflict between two kingdoms could be over a single event:

Such as a catastrophe for a negative event, like a plague that invaded a kingdom and incited a war. 

Or 

Such as a profitable economy that flourishes for a positive event in a single kingdom and the other kingdom becomes jealous and starts stealing from the first kingdom, the other kingdom recognizes the theft for what it is and now they declare war against the other. 

And/or there could be a land outside either of the two kingdoms-that is ruled through sheer power and cunning. Remaining independent and "for now" neutral to either kingdom, deciding to just bide their time to strike against either kingdom at the most opportune time. What motive could there be to strike and what might they be waiting for? No outsiders would know.