However,

we are morally bound to use our art in the defence of others. This is a complicated ethical area, even more so than when we are personally attacked. One of the urban myths of martial arts, with enough real instances to back it up, is the practitioner attempting to break up a fist-fight between husband and wife by attacking the husband, only to have the enraged wife attack her would-be rescuer (after which the husband joins in as well).
When one comes upon a fight in progress, neither its cause nor the guilt or innocence of any involved parties is usually apparent, and wading in full bore against one side or the other while ignorant of the facts may later prove to be a mistake. No doubt both sides of the conflict would argue (and probably believe) that theirs was the just cause.