Deuteronomy 20:10

10.When thou goest forth to war. He now teaches that, even in lawful wars, cruelty is to be repressed, and bloodshed to be abstained from as much as possible. He therefore commands that, when they shall have come to take a city, they should first of all exhort its inhabitants to obtain peace by capitulating; and if they should do so, to keep them alive, and to be content with imposing a tribute on them. This principle of equity was naturally implanted in all nations; hence heralds took their rise, (45) nor did they commence a just war without a solemn proclamation. Besides, inasmuch as the word hostis (an enemy) formerly signified a foreigner (peregrinum,) the Romans mitigated by its mildness the sadness of the reality. On this ground they deemed that faith was to be kept with an enemy; and that sentiment of Cicero is worthy of praise, “that wars must not be undertaken except that we may live in unmolested peace.”

But if God would have his people mindful of humanity in the very midst of the din of arms, we may hence infer how greatly displeasing to Him is human bloodshed. Even those whom He has armed with his authority, He would still have disposed to clemency, and He represses their ardor, lest they should stain with blood the swords given them by His permission. How, then, shall it be lawful for a private person to assume the sword for the purpose of killing his brother? We now understand the object of the instructions here given, and how appropriately they are connected with the Sixth Commandment.

The references in the two following sentences are to Cicero, de Off. 1:12, and 11, and 13.