Slitherine

Hey, in Spartan, anytime I send a diplomat to a nation I'm not on good terms with, I'm forbidden
to execute any friendly tasks such as:
*State Gift Money
*Gift Horses
*Buy Iron
Or do anything that would help improve relationships. Yet it allows me to do evil tasks such as:
*Frame Wheat Trader
*Sabotage Grainiries
*Redirect Egyptian Recruiters
*Poison Garrison Waters
Are you not allowed to do nice things for un-kind nations?

Yes you are limited in what you can do based on their relationship to you and your diplomats experience level.

All right, but how did the nations get so angry at mine in the first place? I never even sent a diplomat to their nation or fought them!

Everything you do effects them. If you attack their friends, if you walk through their territory, if you are a different ethnic group etc.

Ok, thanks! Is it possible to view other player's diplomatic stances with other nations?
Would a nation get mad at you just because of your power, like if you control 20 cities
and they believe you could be a threat?

Yes, and also when you are weak they can see an opportunity. Its hard to keep friends!

Diplomacy is rather unpredictable, though. Usually, when I send a diplomat to someone the relationship with them will improve. One time, though, playing as the Boeotians, the relations with every nations I sent diplomats to worsened from the presence of the diplomats. I had to constantly spend fortunes of silver to keep them stable, and perhaps increase relations with 1% every one or two years, until my diplomats had three stars and I had upgraded my embassy enough to more (cost-)effectively improve relations. Only then I could leave them alone, and relations started improving on their own.

Meanwhile, the Ionian Cyclades, with whom I had never had any diplomats, and no contact of any other kind, became my allies.

For all my testing, ethnic group seems to have nothing to do with it, and aggression seems to have nothing to do with it.