Defending wrong person
One must be very careful in coming to the defense of another person; one may aid the wrong person. The person seen striking another may be the victim who is only defending him/herself. Unless it is obvious who the victim is, or you are positive who the victim is, you must be very careful before getting involved in a confrontation. For example, when you see two people fighting, how are you going to know who is in the right? Your first impression might be that a man is beating up another so you come to the defense of the man getting the worst of it. Then you find out, after you are in jail, that the man winning the fight was an off-duty policeman arresting a dangerous criminal and you interfered with the officer in the performance of his duty. If you assist the wrong person, the courts will be sympathetic as long as you used a reasonable interpretation of the circumstances at the time you acted. If it was a reasonable error and no one was seriously injured, the law may excuse your error.
Defense of property
Generally, where only property is involved and the offender makes no threat against a person, the use of force is not justified. If a person approaches you and demands your wallet, but makes no threats whatsoever, you may refuse the request, but you may not use force against the person. Remember there must be the threat of force and/or an overt action of force. In general, the law does not view property as important enough to justify the use of force to protect or regain it. An exception to this general rule is "hot pursuit."
Hot pursuit. If you witness the theft of your property and make a "hot pursuit" to recover it, you may use minimal force to recover the property but you may not cause serious injury. If the thief responds with force, you may use a reasonable amount of force to defend yourself, but you must end the confrontation as soon as possible. If your pursuit is not immediate or you did not see the actual theft, then the "hot pursuit" justification no longer applies.