신명기 23:7의 Halakhah

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 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.'"