http://en.wikipedia.org/wiki/Right_of_self-defense

"The right of self-defense (according to U.S. law) (also called, when it applies to the defense of another, alter ego defense, defense of others, defense of a third person) is the right for civilians acting on their own behalf to engage in a level of violence, called reasonable force or defensive force, for the sake of defending one's own life or the lives of others, including, in certain circumstances, the use of deadly force."

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