Lets look at it this way. The Amygdala and the fight-flight routine were developed in the primitive brain at a time when people had not developed the higher functions that are associated with the neocortex.
For those who are not aware the Amygdala is part of the mammalian, or more primitive area of our brain. This part of us is associated with survival and emotions. The neocortex on the other hand deals with logical thinking and also our wonderful imaginations.
When we are in a situation where there is a threat the amygdala in our primitive ancestor’s brain sent signals to the body preparing it for running or standing and fighting. The same bodily responses were applicable to either the decision to run or stand and fight.
In the first instance adrenaline would flow into the blood stream preparing muscles for action increasing the heart rate and breathing. Other less needed functions would reduce.
This happened in milliseconds.
The body is now stressed and ready for action. This stressor is fear that is a result of the threat; our brain and body are being stretched in response to this danger. Being stretched can be viewed as stress.
So, yes, in my opinion stress comes first in may cases of anxiety because our more complex brain with the neocortex is being stressed in order for the fight-flight routine to be brought into action. Now our amygdala brings the changes about just as quickly but it also reduces our ability to think logically as the higher brain is taken over by the emotional brain.
The other very effective stressor is the wonderful imagination. Our primitive ancestors appear to have had limited imaginations, once the threat had gone their bodies returned to a normal state. Then life continued.
Imagined actions can have the same emotional effect upon our bodies as the real thing.
We are able to imagine a threat and create amazing internal imagery or complex verbal scenarios about potential dangers. When these are recognised by our amygdala the same arousal system is brought into effect. Something that may have stressed us once only provides the fuel for thoughts that bring it back into mind.
Say you had a run in with your boss about being late some time in the past. The next time that you are delayed on your way to work memories of the first run-in come to mind and the original scenario can be imagined. You have created the stress in your head with the same result as if the boss were right there giving you a dressing down.
Anxiety is created in the mind, as there is no direct threat, only an imagined one.
Fear is what happens when there is an immediate danger. Anxiety is what happens when there is a potential or imagined threat.