foothold has 3 distinct synonyms spread across 3 of its 3 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
an area in hostile territory that has been captured and is held awaiting further troops and supplies
“an attempt to secure a bridgehead behind enemy lines”
a place providing support for the foot in standing or climbing
Synonyms, antonyms and the broader and narrower relations for foothold come from Princeton University’s WordNet. Full sources and licences.
an initial accomplishment that opens the way for further developments
“the town became a beachhead in the campaign to ban smoking outdoors”