Not only does it spread to self - protective aggression in many dogs when they feel cornered, but it spreads to other things. (vin.com)
Something not often noticed here is that fear behavior - like running away - is equally unacceptable and just as much part of the cause of protective aggression as barking / lunging toward the threat. (gawker.com)
Protective aggression usually refers to aggression directed toward people or animals that a dog perceives as threats to his family, or pack. (nevadahumanesociety.org)