Both Uses of

intervene
in
The Iliad by Homer - (translated by: Pope)

Thus Homer has his "speaking horses;" and Virgil his "myrtles distilling blood;" where the latter has not so much as contrived the easy intervention of a deity to save the probability.†
Book Pref.

intervention = get involved; or come between

The armies being ready to engage, a single combat is agreed upon between Menelaus and Paris (by the intervention of Hector) for the determination of the war.†
Book 3 *

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."