nationalism has 1 distinct synonym spread across 1 of its 4 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 multiculturalism, internationalism.
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.
love of country and willingness to sacrifice for it
“they rode the same wave of popular patriotism”
the doctrine that your national culture and interests are superior to any other
the aspiration for national independence felt by people under foreign domination
the doctrine that nations should act independently (rather than collectively) to attain their goals
Synonyms, antonyms and the broader and narrower relations for nationalism come from Princeton University’s WordNet. Full sources and licences.