Peace, it's wonderful! Except when the Peace Movement is actually a subterfuge by the warlike and imperialistic Kril to force the Terrans to abandon the planet, leaving it defenceless and ripe for conquest. The Terran diplomats have their usual primary concerns; covering their, ah, flank and not doing anything that might threaten their careers, which means that they're not about to do anything, period.
Once again, it's up to Retief to thwart the would-be conquerors, even if the unthinkable happens, and he's drummed out of the Corps Diplomatique Terrestriennel. And Retief has never been more dangerous to the villains than when he's operating outside the rules of diplomacy.