If your game setup includes "True AI Diplo" and you have met both plemo and civac in-game and they have met each other and the two of them are not already at war and they are not in an enforced peace treaty with each other, there should just be an option when you open a diplo window with plemo, on the side of the screen with all plemo's stuff (near the bottom, I think?) that says, "Declare War on civac" (or "declare war on [whatever civac has named the civ]" - I forget which) - if you're not playing with True AI Diplo turned on or they're already at war or in enforced peace, there's no clear-cut way to request this, but offering to trade your iron or copper or other military resource for your trade partner's identical resource is usually read as "let's be war allies" - and some players then try to convey more-nuanced information by requesting specific cities, other resources, gold, or whatever else comes handy in the hope that it will be correctly interpreted. This frequently results in hilarious (for lurkers) misinterpretations of intent, but is sometimes highly effective.
(I personally would prefer to go back to the long-ago standard of "no using gold as a countdown, no using resources or cities as code for anything; only propose deals that you expect the recipient may accept" - but that hasn't been the standard for a lonnnnnng time now.)
(I personally would prefer to go back to the long-ago standard of "no using gold as a countdown, no using resources or cities as code for anything; only propose deals that you expect the recipient may accept" - but that hasn't been the standard for a lonnnnnng time now.)
