People call many behaviors aggression, and talk about "treating" aggression as if it were a medical entity, like a staph infection. It's as if any display of symptoms confirms the existence of a full-blown case for which known treatments exist.
The archetypal event for me was the TV trainer who cornered and frightened a poodle puppy until it growled at him, and then told the owners the dog was a "loaded gun." There was a loaded gun on the scene, all right, but it wasn't the dog.
I've taken, in practice, to avoiding the term altogether and speaking of all agonistic behavior (the biological term for hostile or fearful displays) as manifestation of fear, with the correct response being a mix of reducing the stimulus, and reinforcing (clicking) more appropriate behaviors, calming signals definitely being among them, and then gradually reinstating the once-frightening stimulus.
Emma Parsons's dog Benny (a golden in my 1998 experimental class) was afraid of many things; I remember seeing him shrinking from a rubber door mat in the place where we worked, and spending some time with Emma clicking him into crossing it with tail up. Confidence in one area (or trained body language in one area or situation) can then leak into other areas, giving you something to click.
I'm presently doing something similar with a horse, who gets antsy and excited in any new place, even just being led from the barn. It's fear...of whom or what in her history I don't know or care—and has on occasion escalated into "aggression," including nipping, pawing the ground, fighting the lead line, and even bucking and bolting. Lower your head, relax your ears, click. Can you do it here? Yes. Can you do it there? No? Back up a step, we start again. As Alexandra Kurland (author of Clicker Training for Your Horse ) says, the horse is showing you what it doesn't know yet. Aggression-like behavior is usually just dogs doing the same. IMHO [in my humble opinion].