“The courage of a soldier is heightened by his knowledge of his profession,”
De Re Militari (also Epitoma Rei Militaris), Book I, "The Selection and Training of New Levies"
Context: The courage of a soldier is heightened by his knowledge of his profession, and he only wants an opportunity to execute what he is convinced he has been perfectly taught. (Book 1)
Original
Scientia enim rei bellicae dimicandi nutrit audaciam: nemo facere metuit quod se bene didicisse confidit.
Help us to complete the source, original and additional information
Publius Flavius Vegetius Renatus 16
writer of the Later Roman EmpireRelated quotes

“His profession made him rich and he made his profession respectable.”
Samuel Johnson
About

As quoted in Quotes for the Air Force Logistician, pub. Air Force Logistics Management Agency (2001), p. 115

Journal entry (21 July 1944); later published in The Wartime Journals (1970)
Context: The intense artillery fire has stripped the trees of leaves and branches so that the outline of the coral ridge itself can be seen silhouetted against the sky. Since I have been on Owi Island, at irregular intervals through the night and day, the sound of our artillery bombarding this Japanese stronghold has floated in across the water. This afternoon, I stood on the cliff outside our quarters (not daring to sit on the ground because of the danger of typhus) and watched the shells bursting on the ridge. For weeks that handful of Japanese soldiers, variously estimated at between 250 and 700 men, has been holding out against overwhelming odds and the heaviest bombardment our well-supplied guns can give them.
If positions were reversed and our troops held out so courageously and well, their defense would be recorded as one of the most glorious examples of tenacity, bravery, and sacrifice in the history of our nation. But, sitting in the security and relative luxury of our quarters, I listen to American Army officers refer to these Japanese soldiers as "yellow sons of bitches." Their desire is to exterminate the Jap ruthlessly, even cruelly. I have not heard a word of respect or compassion spoken of our enemy since I came here.
It is not the willingness to kill on the part of our soldiers which most concerns me. That is an inherent part of war. It is our lack of respect for even the admirable characteristics of our enemy — for courage, for suffering, for death, for his willingness to die for his beliefs, for his companies and squadrons which go forth, one after another, to annihilation against our superior training and equipment. What is courage for us is fanaticism for him. We hold his examples of atrocity screamingly to the heavens while we cover up our own and condone them as just retribution for his acts.

“A soldier will return as a hero either with a medal on his chest or a metal in his chest.”
P. 52. https://www.goodreads.com/quotes/10937446-a-soldier-will-return-as-a-hero-either-with-a
“He who whets his steel, whets his courage”
Source: Gates of Fire: An Epic Novel of the Battle of Thermopylae
“The Schoolboy, with his satchel in his hand,
Whistling aloud to bear his courage up.”
Part I, line 58. Compare: "Whistling to keep myself from being afraid", John Dryden, Amphitryon Act iii, scene 1.
The Grave (1743)