Sefer HaChinuch ספר החינוך

Not to seek their peace in a war: That we have been prevented from making peace with Ammon and Moav forever. And this matter is that God commanded us that when we besiege countries that we should request [terms of] peace with them before the war, and as I have written in the Order of Shoftim about the commandment to call out for [terms of] peace in an optional war (Sefer HaChinukh 527). And with Ammon and Moav, we have been prevented from this practice. And about this is it stated (Deuteronomy 23:7), “You shall not seek their peace or their welfare, etc.” And so did they say in Sifrei, “It is implied from that which is stated (Deuteronomy 20:10), ‘When you approach a city to fight against it, etc.’ Is it possible also here? Therefore [the verse] teaches us to say, ‘do not seek their peace.’”

That which I have written in the previous commandment; is from the roots of the commandment — that they were villainous to the point that they are not fitting for peace and welfare. But [rather] cruelty against them is worthy and virtuous, as we said. (See Mishneh Torah, Kings and Wars 6:6.)

The laws of the commandment are included in the simple meaning of the verse and in that which we brought from the language of Sifrei. And this prohibition is practiced by the males, who are the fighters of war. At the time that Israel was on its Land [and] that they were fighting with Ammon and Moav, they were [bound] by this prohibition, from making peace with them. But today, in our times, we do not have the power to fight. And also these nations have already lost their names in the jumbling of Sancheriv, as we mentioned in the previous commandment.