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1640s, "operation of an army in the field," from French campagne "campaign," literally "open country," from Old French champagne "countryside, open country" (suited to military maneuvers), from Late Latin campania "level country" (source of Italian campagna, Spanish campaña, Portuguese campanha), from Latin campus "a field" (see campus). Old armies spent winters in quarters and took to the "open field" to seek battle in summer. Extension of meaning from military to political is American English, 1809.
1701, from campaign (n.). Political sense is from 1801. Related: Campaigned; campaigning.