I seem to have problem with getting my 'Pleased' and 'Friendly' friends to join me in battle! Most of the time its because we dont have the same enemies. Most of the times my friends are enemies with eachother, and pals with MY enemies.
Do I need more diplomatic depth? Like actively be-friend enemies of my enemies? I've noticed that the AI seem less eager to go to war if your not at war with the target urself.
they definitely are easier to bribe to go to war if you're in the war already yourself. even if they're willing to without you starting it, their price tag will go down after you start the fight.
my style is that i don't like for folks to declare war on me in the first place, i'm not very good at wars even when i start 'em. so most folks, including you, probably don't have to go to the extremes that i do *giggle*. i also don't play with 18 civs on the map, that's bound to make it harder in your case.
but the key for me is to make a "you're my friends" list and a "you're the enemy, who cares what you think" list and then try to make sure that my friends are friends with
each other. that's partly so that they don't come demanding that i cancel trades with each other, etc. and partly because i need my friends to stay my friends and help out poor-little-defenseless me if some enemy (who i need us both to dislike) starts to be mean. i don't want them going to war with each other, since they'll ask me to join and i'll get negative modifiers for not helping, yuck.
this can sometimes help you: if i want my friends to be not-so-much friends any more since i decided CivA has to die, but i don't want the -1 you DoWd our friend with CivB, i look at their civics. if CivA is using CivB's favorite civic, that makes CivB like him, and if i can get CivA to swap civics (in BtS, using spies, in other versions bribing is your only option), then CivB instantly likes CivA less, sometimes a drop of 3-6 points. often (but not always) that's enough to take their relationship from "pleased" to cautious"; it's usually enough to knock down "friendly" to "pleased". that's much easier if you're spiritual of course, so that you can change civics to do the bribing/espionage civic-switching without anarchy.
you can try to break up religious blocs / stir up tension with religion to get your friend to hate your enemy. or try to get rid of religious differences that are making your friends be enemies with each other. i gifted churchill education and then liberalism in one game to get him to go FR, since he didn't want me to trade with hatty, and she had the only pigs on the planet. i needed her pigs for health, but i didn't want to get -you traded with our worst enemy with churchill, if he invaded me i was toast! for me in that case it was worth giving away (selling/trading to the rest of them) a monopoly tech.
so you might try stuff like that to encourage your friends to be less buddy-buddy with your enemies.