Deuteronomy 7:3

Neither shalt thou make marriages with them,.... Unless they became proselytes, as Rahab, who was married by Salmon, and so those of other nations, as Ruth the Moabitess, and so any captive taken in war; otherwise it was not lawful, bad consequences have followed upon it, which it is the design of this law to prevent; that is, being snared and drawn aside into idolatry, which was the case of Solomon:

thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son; for, according to the Targum of Jonathan, whosoever marries with them, it is as if he married with their idols: and this law, according to the Jewish writers c, is binding with respect to other nations besides the seven; and whosoever marries any Heathen, of whatsoever nation, is to be beaten.

c Maimon. Hilchot lssure Biah, c. 12. sect. 1.