A criminal-law concept describing conduct, procedure, or legal consequence defined by governing offense or rule elements. It applies only when the legally specified conditions for its use or enforcement are satisfied. It does not apply where required elements or conditions are absent. It materially affects charging, adjudication, detention, or sanction decisions in practice. The concept is generally stable, though elements and penalties may be revised over time.

Principle

Extend the right to defensive action to protect third parties while ensuring the intervener's belief about the need and justification is reasonable; balance encouraging intervention with preventing vigilante excesses.

Demonstration

A bystander sees an individual being attacked and intervenes to pull the attacker off the victim, using force proportional to the threat; courts examine whether the intervener reasonably believed the victim faced imminent unlawful harm.

Misapplication

Intervening based on misperception of events, using excessive force, or acting in defense of someone who was actually the aggressor (without reasonable belief) are improper applications and may trigger liability.

Consequence

A successful defense of others claim yields acquittal or justification for the intervener's actions, can reduce civil liability, and shapes public policy on bystander intervention and duty to assist.

Reversal

The converse is acting as a private prosecutor or vigilante where intervention is motivated by vengeance or where the third party had no right to defend, transforming the intervener's conduct into unlawful aggression.

Boundary

Applies only when the intervener's belief in the third party's right to defense is reasonable; excludes defending unlawful conduct, and is subject to jurisdictional limits on the degree of force and obligations to retreat.

Semantic Tension

Tension between protecting vulnerable persons and preventing unauthorized use of force; also between defense of others and necessity or duress when the intervener faces pressure to act.

Synthesis

Defense of others mirrors self-defense: it allows proportionate force to stop an imminent unlawful attack on another when the intervener reasonably believes such force is necessary and justified.