intervene

Etymology

Back-formation from intervention, and/or from Latin interveniō (“come between”, verb).

verb

1. (intransitive) To become involved in a situation, so as to alter or prevent an action.
   The police had to be called to intervene in the fight.
   Nature film-makers are discouraged from intervening in the events they are attempting to capture on film. 19 November 2018, Aamna Mohdin, “Top film-makers back penguin intervention on Attenborough show”, in The Guardian
2. (intransitive) To occur, fall, or come between, points of time, or events.
   An instant intervened between the flash and the report.
   I hadn't seen him since we were in school, and the intervening years had not been kind to him.
   Even sad vicissitude amus’d his soul; And if a sigh would sometimes intervene, And down his cheek a tear of pity roll, A sigh, a tear, so sweet, he wish’d not to controul. 1794, Ann Radcliffe, chapter 4, in The Mysteries of Udolpho, volume 1, London: G.G. and J. Robinson, page 93
   […] he was prepared to allow long silences to intervene rather than exchange pointless words. 1963, John le Carré, chapter 17, in The Spy Who Came In From the Cold, New York: Coward-McCann, published 1964, page 176
3. (intransitive) To occur or act as an obstacle or delay.
   Nothing intervened to prevent the undertaking.
   I told Ántonia I would come back, but life intervened, and it was twenty years before I kept my promise. 1918, Willa Cather, My Ántonia, Book 5, Chapter 1
4. (transitive, intransitive) To say (something) in the middle of a conversation or discussion between other people, or to respond to a situation involving other people.
   Young Scarfe stared, astounded. “You haven’t met before,” Mrs. Gould intervened. “Mr. Decoud—Mr. Scarfe.” 1904, Joseph Conrad, Nostromo, Part 2, Chapter 4
   “That sounds suspiciously like bigotry to me,” intervened Maitland, sweetening his impertinence with a dimpled smile. 1970, J. G. Farrell, Troubles, New York: Knopf, published 1971, Part 2, p. 409
   They all talked nonstop. That is, if you didn’t intervene. They were accustomed to being interrupted. 2014, Rachel Kushner, chapter 10, in The Flamethrowers, New York: Scribner, page 154
5. (transitive, intransitive) To come between, or to be between, persons or things.
   The Mediterranean intervenes between Europe and Africa.
   1668, Joseph Glanvill, Plus Ultra, or, The Progress and Advancement of Knowledge since the Days of Aristotle, London: James Collins, Chapter 11, p. 79, How defective the Art of Navigation was in elder Times, when they Sailed by the observation of the Stars, is easie to be imagin’d: For in dark weather, when their Pleiades, Helice, and Cynosura were hidden from them by the intervening Clouds, the Mariner was at a loss for his Guide, and exposed to the casual conduct of the Winds and Tides.
   If the profits of the merchant importer or merchant manufacturer were taxed, equality seemed to require that those of all the middle buyers, who intervened between either of them and the consumer, should likewise be taxed. 1776, Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations, London: W. Strahan and T. Cadell, Volume 2, Book 5, Chapter 2, Part 2, Article 4, p. 522
   […] small fields and miniature meadows, separated […] by wild self-sown woodlands of birch, alder, holly, mountain ash, and hazel, that meander through the valley, intervening the different estates with natural sylvan marches […] 1839 September, Thomas De Quincey, “Sketches of Life and Manners; from the Autobiography of an English Opium-Eater: Recollections of Grasmere”, in Tait’s Edinburgh Magazine, volume 6, page 569
   Venters calculated that a mile or more still intervened between them and the riders. 1912, Zane Grey, chapter 22, in Riders of the Purple Sage
   I had begun to eye the door and the intervening furniture, and quickly schemed out the best way of immediate exit. 1979, William Styron, chapter 3, in Sophie’s Choice, New York: Bantam, published 1980, page 82
6. (law) In a suit to which one has not been made a party, to put forward a defense of one's interest in the subject matter.
   an application for leave (i.e. permission) to intervene