The answer to your question is in your statement. RP (Role-play). As there is no game mechanic to declare war on certain nations, more-so to wage war on certain nations, the most effective way to work around that, is through Role-play.
I would give you some examples of some ways in which war and role-play come into play in this game. I would also refer you to the game rule where it pertains to role-play and war.
Example of how war, role-play and cooperation driven team-work plays out:-
viewtopic.php?f=18&t=6611 (Role-play Thread)
viewtopic.php?f=5&t=6589#p96285 (OOC/Planning Thread)
Game rules where it pertains to war and role-play:-
viewtopic.php?f=18&t=6363 (I would suggest breezing through Chapters 21-23 to get a general feel).