"You must not seek their peace or prosperity"Deuteronomy 23:7.

Normally, when embarking upon a battle, we are commanded to first offer the enemy terms for a peace proposal (see Positive Commandment 190). Not so with the nations of Amon and Moab, whom we may not offer a peace proposal. [Though if they, of their own volition, offer to make peace, we do accept their proposal—provided it meets the conditions outlined in the positive commandment referenced above.]