this is kind of the way
this is belli -
en.wikipedia.org
it is A reason to go to war
edit - an alliance reason would be
casus foederis
casus belli in the paradox games were used iirc to prevent unchecked constant war - and part of the reason they implemented limits on territory acquisition was for game balance reasons. in other words, it would not allow an unjust war at all in the older games iirc... and/or it put a large rep hit on someone declaring unjust war.
if we are going to have aggressive reasons to cause wars (we want that border city) and aggrieved reasons to cause war (we need to get our city back)... then that might be better handled via a rep system of some sort... a badboy rating if you will (preferably with a leaky bucket)
i agree - especially in a war game - where the reasons (good or bad) for going to war or not should be part of the game itself
right now wars start any time for any reason (as a player)... you can recruit people to your kingdom but people leave with no notice (i throw money at them and give them cities from victories with no apparent effect). I am looking forward to them adding some more visibility to war reasoning (as well as other relations issues)
if you are going to have marriages and potential alliances that result from that - then there should be a system that takes that into account during war decision making
(and preferably a screen that we can look at to see the relations with each power and the entanglements that affect those relations)
also - if they don't want something complex - put a button in the game options to remove relations (or whatever part of it that affects the complexity that they are concerned about)