Deuteronomy 7:2-3

7:2 and he delivers them over to you and you attack them, you must utterly annihilate them. Make no treaty with them and show them no mercy! 7:3 You must not intermarry with them. Do not give your daughters to their sons or take their daughters for your sons,

Joshua 23:12-13

23:12 But if you ever turn away and make alliances with these nations that remain near you, and intermarry with them and establish friendly relations with them, 23:13 know for certain that the Lord our God will no longer drive out these nations from before you. They will trap and ensnare you; they will be a whip that tears your sides and thorns that blind your eyes until you disappear from this good land the Lord your God gave you.