WordNet records no synonym and no antonym for peacekeeper. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a member of a military force that is assigned (often with international sanction) to preserve peace in a trouble area
someone who keeps peace
“she's the peacekeeper in that family”
the pistol of a law officer in the old West
Synonyms, antonyms and the broader and narrower relations for peacekeeper come from Princeton University’s WordNet. Full sources and licences.