intervene

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.”

To occur, fall, or come between, points of time, or events.

“An instant intervened between the flash and the report.”

To occur or act as an obstacle or delay.

“Nothing intervened to prevent the undertaking.”

To say (something) in the middle of a conversation or discussion between other people, or to respond to a situation involving other people.

To come between, or to be between, persons or things.

“The Mediterranean intervenes between Europe and Africa.”

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”