The great danger when working a dog in defence drive is, that the same stimuli which cause defence behaviour also cause avoidance behaviour. One part of protection training. The idea that good dogs should only be worked in defence is a dangerous one, which has wrecked many great dogs. With all the different theories exist about aggression, there still is no conclusive proof available whether or not genuine spontaneous aggression exists. The truth is probably that aggression results from all three processes.
Research is available to support all three theories.
For our purposes however, we should concern ourselves less with where aggression comes from and more with what triggers it, what its goal is, and what its biological significance is. So, let's deal with active aggression. Intraspecific aggression is activated by rivals, and by anti-social behaviour. The goal of the drive is to cause avoidance, submission, or worse of the rival. Biological significance is even distribution of a species over available land to reduce the possibility of food shortages and epidemics.