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.
[4] 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.
[6] In the past, one could resist an unlawful arrest and claim self-defense, however the modern trend is to move away from that rule.
[8] The older view is represented by the U.S. Supreme Court case Bad Elk v. United States[9] 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.
[12] Whether the person retreated may, however, be relevant as to the reasonableness of the use of deadly force, where there isn't an explicit statutory law which affirmatively removes the duty.