There are limits to the smiles and scowls of diplomacy. Armies and missiles are not stopped by stiff notes of condemnation. They are held in check by strength and purpose and the promise of swift punishment.
But secondly you say 'society must exact vengeance, and society must punish'. Wrong on both counts. Vengeance comes from the individual and punishment from God.
It is not only the prisoners who grow coarse and hardened from corporal punishment, but those as well who perpetrate the act or are present to witness it.
SORCERY, n. The ancient prototype and forerunner of political influence. It was, however, deemed less respectable and sometimes was punished by torture and death.
If soldiers are punished before they have grown attached to you, they will not prove submissive; and, unless submissive, then will be practically useless. If, when the soldiers have become attached to you, punishments are not enforced, they will still be unless.
If one leads them with administrative measures and uses punishments to make them conform, the people will be evasive, but if one leads them with virtue, they will come up to expectations.