Why should an attack on a military target be classified as an act of terrorism?

Terrorism is typically an act of violence against a population, usually civilian, to achieve some goal of a political or religious theme. This theory is deemed effective by the terrorists because the civilian population is generally considered defenseless against such action… and hence likely to react through fear (or terror).

However, the military is certainly not defenseless. It’s a fighting force not subject to the whim of an individual’s fear. Also, the destruction or compromise of military targets has inherent value when waging a war. How can terrorism be achieved against such a target?