IDIOMS

warning shot

(figuratively, by extension) An action intended to act as a warning.

Example: Used other than as an idiom: A shot fired as a warning..
2005, J. H. Snider, Speak Softly and Carry a Big Stick: How Local TV Broadcasters Exert Political Power, page 487, ISBN 0595347045
  I believe that most members of Congress interpreted these ads as a warning shot rather than an effective grass roots mobilization tool.