WordNet records no synonym and no antonym for weimar republic. 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.
the German republic founded at Weimar in 1919
“The Weimar Republic was overthrown in 1933 and replaced by the Third Reich”
Synonyms, antonyms and the broader and narrower relations for weimar republic come from Princeton University’s WordNet. Full sources and licences.