The Physiology of Combat Stress Reactions

U.S. Air Force photo by Capt. Anna-Marie Wyant
By Mr. Christian Evans, M.A., M.Div., and Mr. Spencer Case, M.S
July 13, 2020

During an intense firefight in Afghanistan, a soldier became confused and disoriented and began recklessly charging up and down the field between his unit and the enemy. The medic found that the soldier’s heart rate and blood pressure were dangerously high, and he was taken to an aid station. After some rest and reassurance, his heart rate slowed and he returned to his normal state. He was returned to duty the next day.

This anecdote from a forward deployed mental health provider is an example of a service member experiencing an extreme combat stress reaction, also known as a CSR. CSRs are a subset of a larger class of stress reactions known as combat and operational stress reactions, or COSRs, which refer to psychobiological reactions that may stem from combat stressors, such as personal injury, witnessing death or killing of combatants, or operational stressors, such as long work hours in extreme temperatures, dangerous work conditions, or non-combat related injuries. COSRs present as a range of signs and symptoms, including fatigue, sleep problems, decreased attention, hypervigilance, social withdrawal, and anxiety, among other reactions and are considered a normal response to combat and military operations. This leads to the question: Why are COSRs (or CSRs) normal?

The answer to this question lies in the phrase “psychobiological reactions.” Our bodies like to maintain a state of homeostasis in which body functions are running optimally. Our hormones, oxygen level, body temperature, etc., are all perfectly balanced. Stressors knock us out of homeostatic balance, and extreme stress can knock us extremely out of balance.

Our autonomic nervous system (ANS) manages the body’s response to stress, and comprises the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS). Life threatening situations activate our ANS and we enter the “defense cascade,” in which the stages of response to stressors and threats are freeze, flight or fight, fright, flag, and faint. The SNS is the activator primarily responsible for our ramped up freeze, flight, and fight stages (power up); the PNS is the shutdown system primarily responsible for our flag and faint stages (power down). Interestingly, the fright stage straddles the line between SNS and PNS systems and can involve both systems at the same time.

During the first three stages, the SNS powers up. First, we freeze; our heart rate slows and we become hyperaware to assess the situation. If we think we are in danger, we produce more adrenaline, and our heart rate and blood pressure increase as we prepare for flight or fight. If we are unable to flee or fight, then the PNS kicks in to power down. In the moments just before the SNS is powering down, but the PNS is powering up, we might be so frightened that we cannot move (fright), despite having a high heart rate and elevated blood pressure. If the threat is still present when the PNS takes over, we might faint. But once the threat passes, our system normalizes and homeostatic balance is restored.

Under extremely stressful conditions, bodies can power up or power down to an extreme. For service members in a combat setting, these reactions can help them survive. But it’s possible the reactions may be so extreme that they impair service members’ readiness, operational performance, and fitness for duty, which may also affect the unit.

Why is understanding the “biological” cause of CSRs important?

This understanding can help providers work more effectively with service members who have experienced past CSRs (e.g., occurred while deployed to Iraq/Afghanistan); present CSRs (e.g., “just happened” during a field op); and future CSRs (e.g., “at risk” due to an upcoming intense deployment or field op). In all of these situations, the critical two-part message to convey to patients is the same: 

  1. CSRs are an automatic, biologically-based survival response to extreme threat
  2. Understanding the CSR science can help to remove any thoughts of “weakness” or stigma associated with experiencing CSRs and seeking help for them

For more information, download our Combat Stress Reactions: Tips for Providers and Symptom Profiles of Combat Stress Reactions factsheets.

Mr. Evans is a public health analyst at the Psychological Health Center of Excellence. He has a Master of Arts in economics and a Master of Divinity.

Mr. Case is a public health analyst at the Psychological Health Center of Excellence. He has a Master of Science in global health.

The views expressed in Clinician's Corner blogs are solely those of the author and do not necessarily reflect the opinion of the Psychological Health Center of Excellence or Department of Defense.


  • Based on my clinical research on combat trauma, I see repeated combat exposure to be an expression of complete insanity. The sights, sounds and smells of combat are deeply internalized. One of my published articles is titled: "The Dark Beast Behind Combat Trauma". It is my clinical analysis of the book "Shooting Ghosts". I am willing to share copies of my work w/ PHCoE.

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