Thou shalt not seek their peace nor their prosperity all thy days for ever. Not that they were to retain malice towards them, or indulge a spirit of revenge, or not do them any good offices in a private way, which is contrary to the law of love; nor does this contradict any offices of kindness and friendship that might be performed in a personal way: so we find that David had a kindness for Hanun the Ammonite, and showed it, though ill rewarded for it, yet is not blamed for doing it; 2Sa 10:2 for these words respect not persons in a private capacity, but the people of Israel as a body politic, who, as such, were not to carry on trade and commerce with those people, nor intermarry with them, nor make leagues and enter into alliances with them; the reason of which was, because being very near neighbours to them, had there not been such a law, as a wall of partition between them, they might have become very familiar, and so have learnt their evil ways and customs, which this was designed to prevent: the Jews restrain this to overtures and proclamations of peace, which they were not to make with these nations, as they were directed to do when they went out to war with others, De 20:10.
Hilchot Melachim, c. 6. sect.