Lethal force

Deadly force, as defined by the United States Armed Forces, is force that a person uses causing, or that a person knows or should know would create a substantial risk of causing, death or serious bodily harm or injury. In most jurisdictions, 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-traditional weapons in an offensive manner, such as a baseball bat, sharp pencil, tire iron or other, may also be considered deadly force.

In the United States, the use of deadly force by sworn law enforcement officers is lawful when the officer reasonably believes the subject poses a significant threat of serious bodily injury or death to themselves or others. The use of deadly force by law enforcement is also lawful when used to prevent the escape of a fleeing felon when the officer believes escape would pose a significant threat of serious bodily injury or death to members of the public. Common law allowed officers to use any force necessary to effect a felony arrest but this was narrowed in the Tennessee v. Garner ruling in 1985 when the U.S. Supreme Court 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 bodily harm to the officer or others."

In the 1989 Graham v. Connor ruling, the Supreme Court expanded its definition to include "objective reasonableness" standard—not subjective as to what the officer's intent might have been—and it must be judged from the perspective of a reasonable officer at the scene—and its calculus must embody the fact that police officers are often forced to make split-second decisions about the amount of force necessary in a particular situation.

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