Someone who refuses to fight in a war because they think it's morally or spiritually wrong is a conscientious objector.
Some conscientious objectors serve in the military but take on roles that don't require them to fight or carry weapons: working as medics, in offices, or as chaplains. Others, who have ethical objections to the armed forces themselves, refuse to participate at all. The term's first military-related use was during World War I. Conscientious comes from a Latin root meaning "moral sense." Objector is from the verb object, "raise an argument against."
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