Deuteronomy 23:6

Thou shalt not seek their peace That is, make no contracts, either by marriages, or leagues, or commerce with them; but rather constantly keep a jealous eye over them, as enemies who will watch every opportunity to insnare or disturb thee. This counsel was now the more necessary, because a great part of the Israelites lived beyond Jordan in the borders of those people, and therefore God sets up this wall of partition between them, as well knowing the mischief caused by bad neighbours, and Israel's proneness to receive infection from them. Individual Israelites were not hereby forbidden to perform any office of humanity to them, but the body of the nation are forbidden all familiar conversation with them.