Ooooooooooooh it because they fear to lose the capital!
Well, actually, in Feudal Japan you cannot directly go from war to allied, and practically I've never seen players coordinate for canopeners, nor I think it is really something you worry about.
@redrum I agree that Feudal Japan has several issues and it is kind of silly, also tendentially very hard to balance, due to the linear island shape, but, compared with what we have, it is surely one of the best FFA of TripleA.
Of course, it is playable only if the humans are in the minority, if only for balancing reasons.
I think this problem would be fixed by adding 2 castles in each capital (for a total of 3), which I actually believe would be good anyways, giving some anti-stomping margin. If I would do this change (only), would you merge it?
This is a 20 rounds savegame with the 3 castles setup; it looks fine enough:
0_1524681495796_20180425_Feudal Japan_AI(Fast)_02.tsvg