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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.
During the 18th century, a warning shot (in nautical terms, often called a shot across the bow) could be fired towards any ship whose "colours" (nationality) had to be ascertained. According to the law of the sea, a ship thus hailed had to fly her flag and confirm it with a gunshot. Warning shots may still be used in modern times to signal a vessel to stop or keep off and may be fired from other ships, boats, or aircraft.
Warning shots are also used in military aviation, to demand some action of an unresponsive and presumed hostile aircraft; the most common demand would be for the aircraft to change course. The ostensible justification for firing shots is that tracer rounds are very bright and would immediately gain the attention of a crew whose radio is non-functioning, or who might not have noticed radio transmissions. The objective of warning shots is to demonstrate the ability to shoot, and threaten the crew of the unresponsive aircraft that they will be shot down if they do not comply.
Threat to shoot
On the ground, a warning shot from a pistol, revolver, rifle, or shotgun is fired into the air or a nearby object, or aimed so that the shot harmlessly passes by the person being confronted. This is a sufficiently aggressive act to demand attention and alert onlookers that they might be shot if directions are not followed.
Warning shots are not advisable in the civilian sector as a shot fired into the air or other object has the ability to strike an unintended target or ricochet. Warning shots are specifically contraindicated in almost every civilian firearms training course for this reason.
In the United States, firing a warning shot is also problematic for legal reasons, as it may be deemed that the situation was not dangerous enough to justify the use of deadly force, and the shooter may be charged with an attempted murder, reckless discharge of a firearm or unlawful use of a deadly weapon. A notable example of that was the Marissa Alexander case.