defend has 11 distinct synonyms spread across 6 of its 7 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 attack, prosecute.
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.
fight against or resist strongly
“The senator said he would oppose the bill”
Synonyms, antonyms and the broader and narrower relations for defend come from Princeton University’s WordNet. Full sources and licences.