Someone who opposes war even when it is considered impractical to do so; a strict pacifist(noun)
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Use "peacemonger" in a sentence
"• And finally, we have been helping the veteran peacemonger and all-round activist troublemaker Pat Arrowsmith to go straight."
"It set Tony Blair up in a post-prime ministerial career as a part-time peacemonger (though even his friend Lord Levy now feels he has failed in that)."
"The first thirteen parts of the fourteen-part message—which the Americans decoded faster than the Japanese embassy–contained a long and resentful retelling of recent events, with Japan and America in the respective roles of misunderstood peacemonger and arrogant imperialist."