This can be a complicated subject because the dog can display so many subtle signs as it approaches a threshold. Learning to read these signs can take a lifetime of work. As we progress through training it will become more and more critical for the helper to understand the dogs thresholds for stress. The term fight drive is one of the most misused terms in dog training today. What I consider fight drive goes by different terms depending on who you talk to.
Some people call fight drive aggression or active aggression or even defense.
Fight drive is the willingness of a dog to fight a human. It is a factor of experience and genetics that results in a very self confident dog that is always prepared to bring the fight to the man. Unlike defense, fight drive is a very comfortable place for a dog to be. It's a combination of prey, defense, and the experience gained through training or real life encounters. A dog with fight drive knows that it can beat any man, under any circumstance, and in every environment.