All 8 Uses of

intervene
in
The Magic Mountain

He spoke of an unforeseen vexation, of misgivings that had proved justified, of the necessity, on good medical advice, of spending a part of the winter, and perhaps all of it, up here, since cases such as his own were often more stubborn than those that began more spectacularly and since the important thing, really, was to intervene decisively and so arrest his case's progress for good and all.†
Chpt 5.3

intervene = get involved or come between

The thunderstorm cleared, and Frau Chauchat intervened as well, calling her traveling companion's attention to the general decline in conviviality.†
Chpt 7.3 *

intervened = got involved or came between

Every day fierce arguments, out-of-control shouting-matches would erupt between individuals and among entire groups; but the distinguishing mark was that bystanders, instead of being disgusted by those caught up in it or trying to intervene, found their sympathies aroused and abandoned themselves emotionally to the frenzy.†
Chpt 7.9

intervene = get involved or come between

With an anxious start, she reaches for it; he, too, is unnerved, is pulled up halfway from his chair, and is about to dash blindly across eight yards and around an intervening table to come to her aid, as if it would be a catastrophe for her napkin to touch the ground.†
Chpt 4.9 *

And as always Hans Castorp had glanced past Settembrini—who sat with his back to him at one end of the intervening crosswise table—to get a view of the Good Russian table; this time, however, he had paid particular notice to the way her head moved as she spoke, to the arch of her neck and the limp posture of her back.†
Chpt 4.10

Joachim, who attached definite importance to such tokens, had of course seen to it that a few flowers adorned the returnee's place setting; but Hans Castorp's tablemates greeted him with little ceremony, with essentially no more interest than usual, as if he had been gone for three hours and not three weeks—not so much out of indifference to this ordinary, sympathetic fellow or out of self-absorption and preoccupation with their own interesting bodies, but because they were oblivious to the intervening time.†
Chpt 5.2

Then, as prescribed, they promenaded to the bench beside the wooden trough, and afterward James Tienappel enjoyed his first hour of rest cure, to which practice he was introduced by his nephew, who supplemented the plaid roll James had brought along with a camel-hair blanket of his own—one being more than enough for Hans Castorp, given the lovely autumn weather—and instructed his uncle, step by step, in the traditional art of wrapping oneself; in fact, once the consul had been transformed into a smooth, cylindrical mummy, Hans Castorp undid it all, in order to have his uncle repeat the whole established procedure on his own, intervening only occasionally to improve his technique.†
Chpt 6.5

People of Herr Settembrini's sort, the doubters and critics, had observed this fact and seen to it that the old naive form of justice was replaced by the Inquisition, which no longer depended on God's intervening on behalf of truth, but was aimed at obtaining the truth through the confession of the accused.†
Chpt 6.6

Definitions:

(1) (intervene as in: intervened in the war)
the process of getting involved to influence an outcome
The exact meaning of intervention can depend upon its context. For example:
- "intervention program for at-risk youth" -- a process of trying to influence the direction of someone's life
- "medical intervention" -- action taken to improve a medical outcome
- "military intervention" -- interference by a government in affairs of another government (in this case to interfere militarily)
- "had a family intervention" -- an organized meeting of family members to encourage someone (who is often surprised) to recognize and work at solving a problem such as substance abuse
(2) (intervene as in: in the intervening years)
something between two other things -- especially the passage of time between two events
(3) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much more rarely, intervene can mean:
in various senses, including:
- to verbally interrupt -- as in "Not now!' she intervened."
- to be physically between -- as in: "You can't walk from one hill to the other because an impassable river intervenes."