WordNet records no synonym and no antonym for sixth crusade. 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.
a Crusade from 1228 to 1229 led by the Holy Roman Emperor Frederick II who fell ill and was excommunicated by the Pope; by negotiation Frederick II was able to crown himself king of Jerusalem
Synonyms, antonyms and the broader and narrower relations for sixth crusade come from Princeton University’s WordNet. Full sources and licences.