Deadly force

Deadly force, as defined by the United States Armed Forces, is the force which a person uses, causing—or that a person knows, or should know, would create a substantial risk of causing—death or serious bodily harm. The use of deadly force is justified only under conditions of extreme necessity as a last resort, when all lesser means have failed or cannot reasonably be employed.

Firearms, bladed weapons, explosives, and vehicles are among those weapons the use of which is considered deadly force. The use of non-weapons in an aggressive manner, such as a baseball bat or tire iron, may also be considered deadly force.

"Use of deadly force" is often granted to police forces when the person or persons in question are believed to be an immediate danger to people around them. For example, an armed man in a shopping mall shooting at random without regard to the safety of the people around him, and refusing or being unwilling to negotiate, would likely warrant usage of deadly force, as a means to prevent further danger to the community. In the United States this is governed by Tennessee v. Garner, which said that "deadly force...may not be used unless necessary to prevent the escape and the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others." In Australia, it has recently been proposed that police officers should have this power when a person might in the future pose a threat to others (see Australian Anti-Terrorism Act 2005).

In general, all armed bodies, be they the police or military or some offshoot thereof, have the ability to issue authorization for the usage of such force.

In the United States, a civilian may legally use deadly force when it is considered justifiable homicide, that is to say when the civilian feels their own life, or the lives of their family or those around them are in legitimate and imminent danger. However, self-defense resulting in usage of deadly force by a civilian or civilians against an individual or individuals is often subject to examination by a court if it is unclear whether it was necessary at the point of the offense, and whether any further action on the part of the law needs to be taken.

Legal opinions
Use of deadly force in relation to motor vehicles: In , the U.S. Supreme Court held that a police officer’s attempt to terminate a dangerous high-speed car chase that threatens the lives of innocent bystanders does not violate the Fourth Amendment, even when it places the fleeing motorist at risk of serious injury or death. In the Harris case, Officer Scott applied his police car's push bumper to the rear of the suspect's vehicle, causing the suspect vehicle to lose control and crash, resulting in the fleeing suspect being paralyzed waist down.

Traditionally, intentional contact between vehicles has been characterized as unlawful deadly force, though some U.S. federal appellate cases have mitigated this precedent. In , the Seventh Circuit Court of Appeals ruled that although fatalities may result from intentional collisions between automobiles such fatalities are infrequent, and therefore unlawful deadly force should not be presumed to be the level of force applied in such incidents; however, the Adams case was subsequently called into question by, which in turn was reversed by the U.S. Supreme Court in the Scott v. Harris case discussed above; the extent to which Adams can continue to be relied on is uncertain. In the Adams case, the officer rammed the suspect's vehicle.

In , the Seventh Circuit Court of Appeals recognized this principle but added that collisions between automobiles and motorcycles frequently lead to the death of the motorcyclist, and therefore a presumption that unlawful deadly force was used in such intentional collisions is more appropriate. In the Donovan case, the suspect lost control of his motorcycle and became airborne, crashing into the officer's vehicle, which was parked as part of an intercepting roadblock.