Normally dogs maintain a balance between the two. But at times they listen only to the sympathetic side. In these circumstances, adrenalin pours into the blood, the heart beats faster, blood is transferred from skin to muscles and brain, blood pressure increases, red blood cell production increases, coagulation time decreases, digestion and food storage stops, salivation is restrained, the rectum and bladder do not empty as easily as under normal conditions, stored carbohydrates come out of the liver and flood the blood with sugars, respiratory rate increases dramatically, temperature-regulating mechanisms are activated, hair stands on end, and there is profuse panting. So dogs threaten first, to avoid such a cost. If it can be known who will win the fight before the fight occurs, then no blood needs to be shed, leading them to ritualized combat.
Physical violence is only a last resort.
But the rituals and physical cues must be true indicators of one's ability to fight or else cheaters could win as well. Another problem is that the parasympathetic system battles for control as well. This results in a series of aggressive intention movements and desperate counteractions of flight, resulting in dashing forward, pulling back, crouching, sneering, staring, etc. In the end, these are elaborated into threat rituals and combat dances. Tail wagging is often associated with this internal conflict.