Deuteronomy 23:6

Israel was not to seek, i . e . care for and use means to promote, the welfare of these nations. Individuals, however, of these nations might be naturalized in Israel, and as proselytes enter the congregation, as the case of Ruth proves. It was against the nations, as such, that this ban was directed, and this they had brought on themselves by choosing to be enemies of Israel when they might have been friends and allies.