WordNet records no synonym and no antonym for trench warfare. 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 struggle (usually prolonged) between competing entities in which neither side is able to win
“the hope that his superior campaigning skills would make a difference evaporated in the realization that electioneering had become a form of trench warfare”
a type of armed combat in which the opposing troops fight from trenches that face each other
“instead of the war ending quickly, it became bogged down in trench warfare”
Synonyms, antonyms and the broader and narrower relations for trench warfare come from Princeton University’s WordNet. Full sources and licences.