That idea that fatigue is a human factor in combat seems relatively uncontroversial. Military history is replete with examples of how the limits of human physical and mental endurance have affected the character of fighting and the outcome of battles. Perhaps the most salient aspect of military training is preparing soldiers to deal with the rigors of warfare.

Trevor Dupuy was aware that fatigue has a degrading effect on the effectiveness of troops in combat, but he never was able to study the topic specifically himself. He was aware of other examinations of historical experience that were relevant to the issue.

The effectiveness of a military force declines steadily every day that it is engaged in sustained combat. This is an indication that fear has a physical effect on human beings equitable with severe exertion. S.L.A. Marshall documented this extremely well in a report that he wrote a few years before he died. I shall shortly have more to say about S.L.A. Marshall…

An approximate value for the daily effect of fatigue upon the effectiveness of weapons employment emerged from a HERO study several years ago. There is no question that fatigue has a comparable degrading effect upon the ability of a force to advance. I know of no research to ascertain that effect. Until such research is performed, I have arbitrarily assumed that the degrading effect of fatigue upon advance rates is the same as its degrading effect upon weapons effectiveness. To those who might be shocked at such an assumption, my response is: We know there is an effect; it is better to use a crude approximation of that effect than to ignore it…

During World War II when Colonel S.L.A. Marshall was the Chief Historian of the US European Theater of Operations, he undertook a number of interviews of units just after they had been in combat. After the war, in his book Men Against Fire, Marshall asserted that his interviews revealed that only 15% of US infantry soldiers fired their small arms weapons in combat. This revelation created something of a sensation at the time.

It has since been demonstrated that Marshall did not really have solid, scientific data for his assertion. But those who criticize Marshall for unscholarly, unscientific work should realize that in private life he was an exceptionally good newspaper reporter. His conclusions, based upon his observations, may have been largely intuitive, but I am convinced that they were generally, if not specifically, sound…

One of the few examples of the use of military history in the West in recent years was an important study done at the British Defence Operational Analysis Establishment (DOAE) by David Rowland. An unclassified condensation of that study was published in the June 1986 issue of the Journal of the Royal United Services Institution (RUSI). The article, “Assessments of Combat Degradation,” demonstrates conclusively that, in historical combat, small arms weapons have had only one-seventh to one-tenth of their theoretical effectiveness. Rowland does not attempt to say why this is so, but it is interesting that his value of one-seventh is very close to the S. L. A. Marshall 15% figure. Both values translate into casualty effects very similar to those that have emerged from my own research.

The intent of this post is not to rehash the debate on Marshall. As Dupuy noted above, even if Marshall’s conclusions were not based on empirical evidence, his observations on combat were nevertheless on to something important. (Details on the Marshall debate can be easily found with a Google search. A brief discussion took place on the old TDI Forum in 2007.)

David Rowland also presented a paper on the same topic Dupuy referenced above at the Military Operations Research Society (MORS) MORIMOC II conference in 1989, “Assessment of Combat Performance With Small Arms” He later published a book detailing his research on the subject in 2006, The Stress of Battle: Quantifying Human Performance in Combat, which is very much worth tracking down and reading.

Dupuy provided a basic version of his theoretical combat exhaustion methodology on pages 223-224 in Numbers, Predictions and War: Using History to Evaluate Combat Factors and Predict the Outcome of Battles (Indianapolis; New York: The Bobbs-Merrill Co., 1979).

Rules For Exhaustion Rates, 20th Century*

1. The exhaustion factor (ex) of a fresh unit is 1.0; this is the maximum ex value.
2. At the conclusion of an engagement, a new ex factor will be calculated for each side.
3. A unit in normal offensive or defensive combat has its ex factor reduced by .05 for each consecutive day of combat; the ex factor cannot be less than 0.5.
4. An attacking unit opposed by delaying tactics has its ex factor reduced by 0.05 per day.
5. A defending unit in delay posture neither loses nor gains in its ex factor.
6. A withdrawing unit, not seriously engaged, has its ex factor augmented at the rate of 0.05 per day.
7. An advancing unit in pursuit, and not seriously delayed, neither loses nor gains in its ex factor.
8. For a unit in reserve, or in non-active posture, an exhaustion factor of less than 1.0 is augmented at the rate of .1 per day.
9. When a unit in combat, or recently in combat, is reinforced by a unit at least half of its size (in numbers of men), it adopts the ex factor of the reinforcing unit or—if the ex factor of the reinforcing unit is the same or lower than that of the reinforced—both adopt an ex factor 0.1 higher than that of the reinforced unit at the time of reinforcement, save that an ex factor cannot be greater than 1.0.
10. When a unit in combat, or recently in combat, is reinforced by a unit less than half its size, but not less than one quarter its size, augmentations or modifications of ex factors will be 0.5 times those provided for in paragraph 9, above. When the reinforcing unit is less than one-quarter the size of the reinforced unit, but not less than one-tenth its size, augmentations or modifications of ex factors will be 0.25 times those provided for in paragraph 9, above.

* Approximate reflection of preliminary QJM assessment of effects of casualty and fatigue, WWII engagements. These rates are for division or smaller size; for corps and larger units exhaustion rates are calculated for component divisions and smaller separate units.

EXAMPLES OF APPLICATION

1. A division in continuous offensive combat for five days stays in the line in inactive posture for two days, then resumes the offensive:
   1. Combat exhaustion effect: 1 – (5 x .05) = 0.75;
   2. Recuperation effect: 75 + (2 x .l) = 0.95.
2. A division in defensive posture for fifteen days is ordered to undertake a counterattack:
   1. Combat exhaustion effect: 1 – (15 x .05) =0.25; this is below the minimum ex factor, which therefore applies: 0.5;
   2. Recuperation effect: None; ex factor is 0.5.
3. A division in offensive posture for three days is reinforced by two fresh brigades:
   1. Combat exhaustion effect: 1 – (3 x .05) = 0.85;
   2. Reinforcement effect: Augmentation from 0.85 to 1.0.
4. A division in offensive posture for three days is reinforced by one fresh brigade:
   1. Combat exhaustion effect: 1 – (3 x .05) = 0.85;
   2. Reinforcement effect: 0.5 x augmentation from 0.85 to 1 = 0.93.