It's a Shot Across the Bow

A warning shot is a military and/or police term describing an intentionally harmless artillery shot or gunshot with intent to enact direct compliance and order to a hostile perpetrator or enemy forces. It is recognized as signalling intended confrontations on land, sea, and air. As an analogy, "warning shot" can be said of any action of declaration, especially a demonstration of power, intended or perceived as a last warning before hostile measures.

Sarah Krasley