In total, Marshall wrote over 30 books, including Pork Chop Hill: The American Fighting Man in Action, later made into a film of the same name, as well as The Vietnam Primer, co-authored by Colonel David H. Hackworth.

[1] Why this is so remains contested; Marshall argued that even with their own lives at risk, the resistance of the average individual “...toward killing a fellow man" was such that "he will not...take life if it is possible to turn away from that responsibility and at the vital point, he becomes a conscientious objector".

[7] Shortly after the Armistice, Marshall was selected to take the entrance examinations for the United States Military Academy, part of an initiative to promote exceptional soldiers from the ranks.

[12] Following American entry into World War II in December 1941, the United States Army created the "Center of Military History", whose role was to "gather historically significant data and materials" for the benefit of future historians, an organisation that still exists.

[4][a] Marshall argued civilian norms against taking life were so strong many conscripts could not bring themselves to kill, even at the risk of their own lives, and suggested changes in training that would increase the percentage willing to engage the enemy with direct fire.

[17]) Hundreds of monographs were written based on this data project, including the 1994 publication Anvil of War: German Generalship in Defense of the Eastern Front, edited by Peter G. Tsouras.

[4] The Army Chief of Military History's representative on the tour, Colonel David H. Hackworth, collected his own observations from the trip and published them as The Vietnam Primer, with Marshall credited as co-author.

[19] Although his conclusions were widely accepted during his lifetime, after his death in 1977 Marshall's approach was questioned by scholars such as Roger Spiller and Kelly Jordan, who claimed the interviews used to support his ratio-of-fire theory either did not exist or were fabricated.

"[26] Marshall received similar criticisms from Harold Leinbaugh, an ex-FBI employee and WWII infantry veteran, who considered his conclusions a slur on the fighting ability of American soldiers and labelled them "absurd, ridiculous and totally nonsensical".

[31] Surveys of after action reports conducted during WWII in the British and Soviet armies showed low firing rates were common in both[citation needed]; Russian officers suggested inspecting rifles after combat, and executing those found with clean barrels.

He argued significant parts of Marshall's service record were not substantiated by independent evidence, including his claim to have been the youngest commissioned officer in the US Army or to have commanded troops in combat.

[35] John Marshall ultimately concluded the vast majority of his grandfather's wartime experiences were independently verified by his service record and any exaggerations were minor and did not undermine the validity of his later work.

[14] Much of the ongoing discussion regarding his research centres on reasons for "non-firing" and is of continuing interest to militaries in order to determine how to optimise training and manage issues like post traumatic stress disorder.

[38] Engen suggests Marshall's work led combat psychologists to identify the act of killing as a major factor in PTSD, not just an individual's personal experience under fire or the deaths of their comrades.

[40] Marshall's contention low firing rates were a function of social conditioning against killing has been partially supported by historians like Omer Bartov, who suggests weakening these norms through deliberate brutalisation was one reason for the Wehrmacht's better combat performance in WWII compared to other armies.

Bartov argues achieving this came from the long-standing German military doctrine of wide scale reprisals against civilians or those accused of supporting partisan operations and Nazi propaganda describing opponents as "sub-human".

[41] However, he identified other elements in overcoming this reluctance, the strongest being loyalty to the group; paradoxically, the enormous casualties suffered by the Wehrmacht led to an increased focus on sections of 4–6 'comrades', which were far better at maintaining morale and fighting ability.