Self-defense (United States)

In the United States, self-defense is an affirmative defense that is used to justify the use of force by one person against another person under specific circumstances.

In the U.S., the general rule is that "[a] person is privileged to use such force as reasonably appears necessary to defend him or herself against an apparent threat of unlawful and immediate violence from another." In cases involving non-deadly force, this means that the person must reasonably believe that their use of force was necessary to prevent imminent, unlawful physical harm. When the use of deadly force is involved in a self-defense claim, the person must also reasonably believe that their use of deadly force is immediately necessary to prevent the other's infliction of great bodily harm or death. Most states no longer require a person to retreat before using deadly force although these laws are irreconcilable with international human rights law. In the minority of jurisdictions which do require retreat, there is no obligation to retreat when it is unsafe to do so or when one is inside one's own home.

A person who was the initial aggressor cannot claim self-defense as a justification unless they abandon the combat or the other party has responded with excessive force. If the aggressor has abandoned the combat, they normally must attempt to communicate that abandonment to the other party.

In the past, one could resist an unlawful arrest and claim self-defense, however the modern trend is to move away from that rule. In most jurisdictions allowing a person to resist an unlawful arrest, the state requires that the unlawful arrest be accompanied by excessive force. The older view is represented by the U.S. Supreme Court case Bad Elk v. United States where an off-duty Sioux police officer was granted a new trial after being convicted of killing an on-duty police officer who was attempting to illegally arrest the man, because, at the initial trial, the jury was not instructed that it could convict on a lesser offense, such as manslaughter.

In some jurisdictions, there is an imperfect self-defense rule, where an individual who mistakenly believes that he was justified in using deadly force in self-defense, but is not legally justified, may have a murder conviction reduced to a manslaughter conviction instead.

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