Other forms: intervened, intervening, intervenes
To intervene is to step into something that is already happening and change how it goes. Two children scuffling in the playground until a teacher intervenes; a neighbour who hears shouting through the wall and knocks on the door.
Countries intervene in wars, central banks intervene in markets, a surgeon intervenes when the drugs stop working. The noun is intervention — military intervention, an intervention by the police. Intervene takes IN: you intervene in a dispute, never intervene a dispute. It has a quieter second use for time and events that come between two points: they meant to marry in June, but the war intervened, and in the years that intervened they lost touch. In that sense, whatever intervenes gets in the way.
1verbinterveneTo step into a situation in order to change what happens.
“Police had to intervene before the argument turned into a fight.”
synonyms: step in, interfere
2verbeventOf an event or a period of time, to come between two points and get in the way.
“They had planned to marry that summer, but the war intervened.”
Intervene reached English partly as a back-formation from intervention and partly straight from Latin interveniō, 'come between'. Further back it is traced to the Proto-Indo-European root gʷem-. The picture inside the Latin is worth keeping: something coming in between two things that were heading for each other.
Proto-Indo-European
*gʷem-
Latin
interveniō — come between
English, today
intervene
a back-formation from intervention, and/or from Latin interveniō (“come between”).
Etymology from Wiktionary, CC BY-SA