Friction is the only concept that more or less corresponds to the factors that distinguish real war from war on paper.
Carl von ClausewitzIn War, the young soldier is very apt to regard unusual fatigues as the consquence of faults, mistakes, and embarrassment in the conduct of the whole, and to become distressed and depondent as a consequence. This would not happen if he had been prepared for this beforehand by exercises in peace.
Carl von Clausewitz