i did forget something!
VASSALS CAN MAKE A MESS OF THINGS
the whole "worst enemy" thing is for trading of course. when the AI is deciding whether to trade with you or not, they're evaluating their attitude towards your "team". usually a team is a PA, and/or a team you set up in the custom game screen. but for tech trades, it gets weird, because your team there is defined to also include any vassals you have.
so, how the AI really feels about you for "should i trade or shouldn't i?" calculations changes if you have somebody else on your team. if you do, then he has to be Friendly with
every team member to actually count as Friendly with you. but that isn't reflected on the diplo screen, that screen only shows you how he feels about each individual leader.
that's why sometimes you get hit with "we fear you are becoming too advanced" even tho friendly status over-rides that; vassals can change it so that you're not really at friendly. and it is in theory possible to have so many vassals that an AI hates so much that when he averages out his attitude toward the team, "you" (meaning your team, but it counts vs. you of course) are his worst enemy even tho the diplo screen tells you that he's pleased with you.
the "no worst enemy if cautious or higher with everybody" rule still works tho. if he's not annoyed with anybody then he obviously isn't annoyed with your vassals, so yay! if he's furious with your vassals, you kind of know he's not going to be overjoyed that you're trading with them. but if you don't know about this whole "team" thing (which isn't intuitive, at least to me), it can be really puzzling. it doesn't happen very often, i've never had it in a game, but i've seen screenshots of Shaka telling someone "we will never trade with you, our worst enemy!" while he was pleased with the guy. the player had at least 2, i think 3, vassals. basically, don't go around vassalizing too many jerks if you want to trade with the non-jerks
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you would check me into a mental hospital or something if you saw how often i check the diplo screen during some games. i like to play OCC and go for diplomatic victory without killing off a lot of folks. obviously i'm not going to have enough votes to matter, so i micromanage all my trades and babysit who's getting along with who, etc. and then i vote for my opponent for giggles
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