WordNet records no synonym and no antonym for allies. It is the only member of each of its synsets, 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 alliance of nations that fought the Axis in World War II and which (with subsequent additions) signed the charter of the United Nations in 1945
in World War I the alliance of Great Britain and France and Russia and all the other nations that became allied with them in opposing the Central Powers
Synonyms, antonyms and the broader and narrower relations for allies come from Princeton University’s WordNet. Full sources and licences.
an alliance of nations joining together to fight a common enemy