war has 2 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are peace, make peace.
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.
the waging of armed conflict against an enemy
“thousands of people were killed in the war”
a legal state created by a declaration of war and ended by official declaration during which the international rules of war apply
“war was declared in November but actual fighting did not begin until the following spring”
an active struggle between competing entities
“a price war”
a concerted campaign to end something that is injurious
“the war on poverty”
make or wage war
Synonyms, antonyms and the broader and narrower relations for war come from Princeton University’s WordNet. Full sources and licences.