And I don't want any cop-out answers like "Oh these guys are pacifists cause they make others do the fighting for them", I mean a genuine faction of people with power, who reject war, even when it's convenient for them to do war, and even when going to war would be considered the right thing to do by the masses, out of a genuine principle of pacifism. How do you make those people seem like a convincing faction?
My first thought for a pacifist faction is in science fiction where it's some post scarcity group that is aloof to the Galaxy, above it all, etc. As for making them compelling I'm not sure. Maybe sub factions within that want to become more interventionist pushing back against their isolation.