WordNet records no synonym and no antonym for dirty war. It is the only member of its synset, 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.
an offensive conducted by secret police or the military of a regime against revolutionary and terrorist insurgents and marked by the use of kidnapping and torture and murder with civilians often being the victims
“thousands of people disappeared and were killed during Argentina's dirty war in the late 1970s”
Synonyms, antonyms and the broader and narrower relations for dirty war come from Princeton University’s WordNet. Full sources and licences.