intervention has 4 distinct synonyms spread across 4 of its 5 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 one recorded opposite is nonintervention.
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 act of intervening (as to mediate a dispute, etc.)
“it occurs without human intervention”
a policy of intervening in the affairs of other countries
the act or fact of interposing one thing between or among others
(law) a proceeding that permits a person to enter into a lawsuit already in progress; admission of person not an original party to the suit so that person can protect some right or interest that is allegedly affected by the proceedings
“the purpose of intervention is to prevent unnecessary duplication of lawsuits”
care provided to improve a situation (especially medical procedures or applications that are intended to relieve illness or injury)
Synonyms, antonyms and the broader and narrower relations for intervention come from Princeton University’s WordNet. Full sources and licences.