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?
Larry Niven's Known Space books have the Man-Kzin wars, where humans were pacifist and unarmed when they entered the conflict. At least initially, humans would use ship engines as makeshift weapons based on the theory of "any sufficiently powerful form of propulsion can also be a weapon."